diff --git a/app/modules/billing/services/admin_subscription_service.py b/app/modules/billing/services/admin_subscription_service.py
index 6a334a8e..8357a0ad 100644
--- a/app/modules/billing/services/admin_subscription_service.py
+++ b/app/modules/billing/services/admin_subscription_service.py
@@ -119,7 +119,8 @@ class AdminSubscriptionService:
 
         if active_subs > 0:
             raise BusinessLogicException(
-                f"Cannot delete tier: {active_subs} active subscriptions are using it"
+                f"Cannot delete tier: {active_subs} active subscriptions are using it",
+                "TIER_HAS_ACTIVE_SUBSCRIPTIONS",
             )
 
         tier.is_active = False
diff --git a/app/modules/billing/tests/unit/test_admin_subscription_service.py b/app/modules/billing/tests/unit/test_admin_subscription_service.py
new file mode 100644
index 00000000..74411309
--- /dev/null
+++ b/app/modules/billing/tests/unit/test_admin_subscription_service.py
@@ -0,0 +1,642 @@
+# app/modules/billing/tests/unit/test_admin_subscription_service.py
+"""Unit tests for AdminSubscriptionService (admin tier & subscription management)."""
+
+from datetime import UTC, datetime, timedelta
+
+import pytest
+
+from app.exceptions import (
+    BusinessLogicException,
+    ConflictException,
+    ResourceNotFoundException,
+)
+from app.modules.billing.exceptions import TierNotFoundException
+from app.modules.billing.models import (
+    BillingHistory,
+    MerchantSubscription,
+    SubscriptionStatus,
+    SubscriptionTier,
+)
+from app.modules.billing.services.admin_subscription_service import (
+    AdminSubscriptionService,
+)
+from app.modules.tenancy.models import Merchant
+
+
+# ============================================================================
+# Tier Management
+# ============================================================================
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestAdminGetTiers:
+    """Tests for get_tiers with filtering."""
+
+    def setup_method(self):
+        self.service = AdminSubscriptionService()
+
+    def test_get_tiers_active_only_by_default(self, db, admin_billing_tiers):
+        """Default call returns only active tiers."""
+        tiers = self.service.get_tiers(db)
+
+        assert len(tiers) == 3
+        assert all(t.is_active for t in tiers)
+
+    def test_get_tiers_include_inactive(self, db, admin_billing_tiers):
+        """include_inactive=True returns all tiers."""
+        admin_billing_tiers[0].is_active = False
+        db.flush()
+
+        tiers = self.service.get_tiers(db, include_inactive=True)
+        assert len(tiers) == 3
+
+    def test_get_tiers_active_excludes_inactive(self, db, admin_billing_tiers):
+        """Active filter excludes inactive tiers."""
+        admin_billing_tiers[0].is_active = False
+        db.flush()
+
+        tiers = self.service.get_tiers(db, include_inactive=False)
+        assert len(tiers) == 2
+        assert all(t.code != "essential" for t in tiers)
+
+    def test_get_tiers_ordered_by_display_order(self, db, admin_billing_tiers):
+        """Tiers are returned in display_order."""
+        tiers = self.service.get_tiers(db)
+        orders = [t.display_order for t in tiers]
+        assert orders == sorted(orders)
+
+    def test_get_tiers_platform_filter(self, db, test_platform, admin_billing_tiers):
+        """Platform filter returns platform-specific + global tiers."""
+        admin_billing_tiers[0].platform_id = test_platform.id
+        db.flush()
+
+        tiers = self.service.get_tiers(db, platform_id=test_platform.id)
+        codes = {t.code for t in tiers}
+        assert "essential" in codes  # Platform-specific
+        assert "professional" in codes  # Global
+        assert "business" in codes  # Global
+
+    def test_get_tiers_platform_filter_excludes_other(
+        self, db, test_platform, another_platform, admin_billing_tiers
+    ):
+        """Tiers from another platform are excluded."""
+        admin_billing_tiers[0].platform_id = another_platform.id
+        db.flush()
+
+        tiers = self.service.get_tiers(db, platform_id=test_platform.id)
+        assert all(t.code != "essential" for t in tiers)
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestAdminGetTierByCode:
+    """Tests for get_tier_by_code."""
+
+    def setup_method(self):
+        self.service = AdminSubscriptionService()
+
+    def test_get_tier_by_code_found(self, db, admin_billing_tiers):
+        """Returns the tier when it exists."""
+        tier = self.service.get_tier_by_code(db, "professional")
+        assert tier.code == "professional"
+        assert tier.name == "Professional"
+
+    def test_get_tier_by_code_not_found_raises(self, db):
+        """Raises TierNotFoundException for nonexistent code."""
+        with pytest.raises(TierNotFoundException):
+            self.service.get_tier_by_code(db, "nonexistent")
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestAdminCreateTier:
+    """Tests for create_tier."""
+
+    def setup_method(self):
+        self.service = AdminSubscriptionService()
+
+    def test_create_tier_success(self, db):
+        """Creates a new tier with provided data."""
+        data = {
+            "code": "starter",
+            "name": "Starter",
+            "description": "A starter plan",
+            "price_monthly_cents": 1900,
+            "price_annual_cents": 19000,
+            "display_order": 0,
+            "is_active": True,
+            "is_public": True,
+        }
+        tier = self.service.create_tier(db, data)
+        db.flush()
+
+        assert tier.code == "starter"
+        assert tier.name == "Starter"
+        assert tier.price_monthly_cents == 1900
+
+    def test_create_tier_duplicate_raises(self, db, admin_billing_tiers):
+        """Duplicate tier code raises ConflictException."""
+        data = {
+            "code": "essential",
+            "name": "Essential Duplicate",
+            "price_monthly_cents": 4900,
+            "display_order": 1,
+            "is_active": True,
+            "is_public": True,
+        }
+        with pytest.raises(ConflictException, match="already exists"):
+            self.service.create_tier(db, data)
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestAdminUpdateTier:
+    """Tests for update_tier."""
+
+    def setup_method(self):
+        self.service = AdminSubscriptionService()
+
+    def test_update_tier_name(self, db, admin_billing_tiers):
+        """Updates tier fields."""
+        tier = self.service.update_tier(db, "essential", {"name": "Essential Plus"})
+        assert tier.name == "Essential Plus"
+
+    def test_update_tier_price(self, db, admin_billing_tiers):
+        """Updates tier pricing."""
+        tier = self.service.update_tier(
+            db, "essential", {"price_monthly_cents": 5900}
+        )
+        assert tier.price_monthly_cents == 5900
+
+    def test_update_tier_nonexistent_raises(self, db):
+        """Updating a nonexistent tier raises."""
+        with pytest.raises(TierNotFoundException):
+            self.service.update_tier(db, "nonexistent", {"name": "X"})
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestAdminDeactivateTier:
+    """Tests for deactivate_tier (soft-delete)."""
+
+    def setup_method(self):
+        self.service = AdminSubscriptionService()
+
+    def test_deactivate_tier_sets_inactive(self, db, admin_billing_tiers):
+        """Deactivation sets is_active to False."""
+        self.service.deactivate_tier(db, "business")
+        db.flush()
+
+        tier = db.query(SubscriptionTier).filter(
+            SubscriptionTier.code == "business"
+        ).first()
+        assert tier.is_active is False
+
+    def test_deactivate_tier_with_active_subs_raises(
+        self, db, admin_billing_tiers, admin_billing_subscription
+    ):
+        """Cannot deactivate a tier used by active subscriptions."""
+        with pytest.raises(BusinessLogicException, match="active subscriptions"):
+            self.service.deactivate_tier(db, "essential")
+
+    def test_deactivate_tier_with_cancelled_subs_ok(
+        self, db, admin_billing_tiers, admin_billing_subscription
+    ):
+        """Can deactivate a tier used only by cancelled subscriptions."""
+        admin_billing_subscription.status = SubscriptionStatus.CANCELLED.value
+        db.flush()
+
+        # Should not raise
+        self.service.deactivate_tier(db, "essential")
+
+    def test_deactivate_nonexistent_tier_raises(self, db):
+        """Deactivating a nonexistent tier raises."""
+        with pytest.raises(TierNotFoundException):
+            self.service.deactivate_tier(db, "nonexistent")
+
+
+# ============================================================================
+# Subscription Management
+# ============================================================================
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestAdminListSubscriptions:
+    """Tests for list_subscriptions with pagination and filtering."""
+
+    def setup_method(self):
+        self.service = AdminSubscriptionService()
+
+    def test_list_subscriptions_empty(self, db):
+        """Returns empty results when no subscriptions exist."""
+        result = self.service.list_subscriptions(db)
+        assert result["total"] == 0
+        assert result["results"] == []
+        assert result["page"] == 1
+
+    def test_list_subscriptions_returns_results(
+        self, db, admin_billing_subscription
+    ):
+        """Returns subscriptions joined with merchant."""
+        result = self.service.list_subscriptions(db)
+        assert result["total"] == 1
+        sub, merchant = result["results"][0]
+        assert isinstance(sub, MerchantSubscription)
+        assert isinstance(merchant, Merchant)
+
+    def test_list_subscriptions_filter_by_status(
+        self, db, admin_billing_subscription, admin_billing_subscription_trial
+    ):
+        """Status filter returns only matching subscriptions."""
+        result = self.service.list_subscriptions(db, status="trial")
+        assert result["total"] == 1
+        sub, _ = result["results"][0]
+        assert sub.status == "trial"
+
+    def test_list_subscriptions_filter_by_tier(
+        self, db, admin_billing_tiers, admin_billing_subscription
+    ):
+        """Tier filter returns only matching subscriptions."""
+        result = self.service.list_subscriptions(db, tier="essential")
+        assert result["total"] == 1
+
+        result = self.service.list_subscriptions(db, tier="professional")
+        assert result["total"] == 0
+
+    def test_list_subscriptions_search_by_merchant_name(
+        self, db, admin_billing_subscription, test_merchant
+    ):
+        """Search filters by merchant name (case-insensitive)."""
+        # test_merchant has a name like "Test Merchant xxxxxxxx"
+        result = self.service.list_subscriptions(db, search="Test Merchant")
+        assert result["total"] == 1
+
+        result = self.service.list_subscriptions(db, search="NonExistent Corp")
+        assert result["total"] == 0
+
+    def test_list_subscriptions_pagination(
+        self, db, admin_billing_multiple_subscriptions
+    ):
+        """Pagination returns correct page with correct count."""
+        result = self.service.list_subscriptions(db, page=1, per_page=2)
+        assert result["total"] == 3
+        assert len(result["results"]) == 2
+        assert result["pages"] == 2
+
+        result2 = self.service.list_subscriptions(db, page=2, per_page=2)
+        assert len(result2["results"]) == 1
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestAdminGetSubscription:
+    """Tests for get_subscription."""
+
+    def setup_method(self):
+        self.service = AdminSubscriptionService()
+
+    def test_get_subscription_found(self, db, admin_billing_subscription):
+        """Returns (subscription, merchant) tuple."""
+        sub, merchant = self.service.get_subscription(
+            db,
+            admin_billing_subscription.merchant_id,
+            admin_billing_subscription.platform_id,
+        )
+        assert sub.id == admin_billing_subscription.id
+        assert isinstance(merchant, Merchant)
+
+    def test_get_subscription_not_found_raises(self, db):
+        """Raises ResourceNotFoundException when not found."""
+        with pytest.raises(ResourceNotFoundException):
+            self.service.get_subscription(db, 99999, 99999)
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestAdminUpdateSubscription:
+    """Tests for update_subscription — the tier_code→tier_id resolution bug area."""
+
+    def setup_method(self):
+        self.service = AdminSubscriptionService()
+
+    def test_update_subscription_status(self, db, admin_billing_subscription):
+        """Can update subscription status."""
+        sub, _ = self.service.update_subscription(
+            db,
+            admin_billing_subscription.merchant_id,
+            admin_billing_subscription.platform_id,
+            {"status": SubscriptionStatus.PAST_DUE.value},
+        )
+        assert sub.status == SubscriptionStatus.PAST_DUE.value
+
+    def test_update_subscription_tier_code_resolves_to_tier_id(
+        self, db, admin_billing_tiers, admin_billing_subscription
+    ):
+        """
+        tier_code in update_data is resolved to tier_id (FK).
+
+        This is the exact bug that was fixed in commit d1fe358 — the admin form
+        sends tier_code (string) but the model uses tier_id (int FK).
+        """
+        sub, _ = self.service.update_subscription(
+            db,
+            admin_billing_subscription.merchant_id,
+            admin_billing_subscription.platform_id,
+            {"tier_code": "professional"},
+        )
+        assert sub.tier_id == admin_billing_tiers[1].id
+
+    def test_update_subscription_tier_code_nonexistent_raises(
+        self, db, admin_billing_subscription
+    ):
+        """Nonexistent tier_code raises TierNotFoundException."""
+        with pytest.raises(TierNotFoundException):
+            self.service.update_subscription(
+                db,
+                admin_billing_subscription.merchant_id,
+                admin_billing_subscription.platform_id,
+                {"tier_code": "nonexistent"},
+            )
+
+    def test_update_subscription_not_found_raises(self, db):
+        """Updating a nonexistent subscription raises."""
+        with pytest.raises(ResourceNotFoundException):
+            self.service.update_subscription(db, 99999, 99999, {"status": "active"})
+
+
+# ============================================================================
+# Billing History
+# ============================================================================
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestAdminListBillingHistory:
+    """Tests for list_billing_history."""
+
+    def setup_method(self):
+        self.service = AdminSubscriptionService()
+
+    def test_list_billing_history_empty(self, db):
+        """Returns empty when no records exist."""
+        result = self.service.list_billing_history(db)
+        assert result["total"] == 0
+        assert result["results"] == []
+
+    def test_list_billing_history_with_records(
+        self, db, admin_billing_history_records
+    ):
+        """Returns billing history joined with merchant."""
+        result = self.service.list_billing_history(db)
+        assert result["total"] == 3
+        bh, merchant = result["results"][0]
+        assert isinstance(bh, BillingHistory)
+        assert isinstance(merchant, Merchant)
+
+    def test_list_billing_history_filter_by_merchant(
+        self, db, admin_billing_history_records, test_merchant
+    ):
+        """Merchant filter returns only matching records."""
+        result = self.service.list_billing_history(
+            db, merchant_id=test_merchant.id
+        )
+        assert result["total"] == 3
+
+        result = self.service.list_billing_history(db, merchant_id=99999)
+        assert result["total"] == 0
+
+    def test_list_billing_history_filter_by_status(
+        self, db, admin_billing_history_records
+    ):
+        """Status filter returns matching records."""
+        result = self.service.list_billing_history(db, status="paid")
+        assert result["total"] == 3
+
+        result = self.service.list_billing_history(db, status="failed")
+        assert result["total"] == 0
+
+    def test_list_billing_history_pagination(
+        self, db, admin_billing_history_records
+    ):
+        """Pagination works correctly."""
+        result = self.service.list_billing_history(db, page=1, per_page=2)
+        assert result["total"] == 3
+        assert len(result["results"]) == 2
+        assert result["pages"] == 2
+
+
+# ============================================================================
+# Statistics
+# ============================================================================
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestAdminGetStats:
+    """Tests for get_stats (dashboard analytics)."""
+
+    def setup_method(self):
+        self.service = AdminSubscriptionService()
+
+    def test_get_stats_empty_db(self, db):
+        """Stats with no subscriptions return zeroes."""
+        stats = self.service.get_stats(db)
+
+        assert stats["total_subscriptions"] == 0
+        assert stats["active_count"] == 0
+        assert stats["trial_count"] == 0
+        assert stats["mrr_cents"] == 0
+        assert stats["arr_cents"] == 0
+        assert stats["tier_distribution"] == {}
+
+    def test_get_stats_counts_by_status(
+        self, db, admin_billing_subscription, admin_billing_subscription_trial
+    ):
+        """Status counts are accurate."""
+        stats = self.service.get_stats(db)
+
+        assert stats["total_subscriptions"] == 2
+        assert stats["active_count"] == 1
+        assert stats["trial_count"] == 1
+
+    def test_get_stats_tier_distribution(
+        self, db, admin_billing_tiers, admin_billing_subscription
+    ):
+        """Tier distribution counts active/trial subscriptions by tier name."""
+        stats = self.service.get_stats(db)
+
+        assert "Essential" in stats["tier_distribution"]
+        assert stats["tier_distribution"]["Essential"] == 1
+
+    def test_get_stats_mrr_calculation_monthly(
+        self, db, admin_billing_tiers, admin_billing_subscription
+    ):
+        """MRR for a monthly subscription equals the monthly price."""
+        stats = self.service.get_stats(db)
+        # Essential tier is 4900 cents/month
+        assert stats["mrr_cents"] == 4900
+        assert stats["arr_cents"] == 4900 * 12
+
+    def test_get_stats_mrr_calculation_annual(
+        self, db, admin_billing_tiers, admin_billing_subscription
+    ):
+        """MRR for an annual subscription is annual_price / 12."""
+        admin_billing_subscription.is_annual = True
+        db.flush()
+
+        stats = self.service.get_stats(db)
+        # Essential annual = 49000 cents → MRR = 49000 / 12 = 4083
+        assert stats["mrr_cents"] == 49000 // 12
+        assert stats["arr_cents"] == 49000
+
+
+# ============================================================================
+# Fixtures
+# ============================================================================
+
+
+@pytest.fixture
+def admin_billing_tiers(db):
+    """Create essential, professional, business tiers for admin tests."""
+    tiers = [
+        SubscriptionTier(
+            code="essential",
+            name="Essential",
+            price_monthly_cents=4900,
+            price_annual_cents=49000,
+            display_order=1,
+            is_active=True,
+            is_public=True,
+        ),
+        SubscriptionTier(
+            code="professional",
+            name="Professional",
+            price_monthly_cents=9900,
+            price_annual_cents=99000,
+            display_order=2,
+            is_active=True,
+            is_public=True,
+        ),
+        SubscriptionTier(
+            code="business",
+            name="Business",
+            price_monthly_cents=19900,
+            price_annual_cents=199000,
+            display_order=3,
+            is_active=True,
+            is_public=True,
+        ),
+    ]
+    db.add_all(tiers)
+    db.commit()
+    for t in tiers:
+        db.refresh(t)
+    return tiers
+
+
+@pytest.fixture
+def admin_billing_subscription(db, test_merchant, test_platform, admin_billing_tiers):
+    """Create an active subscription on the essential tier."""
+    now = datetime.now(UTC)
+    sub = MerchantSubscription(
+        merchant_id=test_merchant.id,
+        platform_id=test_platform.id,
+        tier_id=admin_billing_tiers[0].id,
+        status=SubscriptionStatus.ACTIVE.value,
+        period_start=now,
+        period_end=now + timedelta(days=30),
+    )
+    db.add(sub)
+    db.commit()
+    db.refresh(sub)
+    return sub
+
+
+@pytest.fixture
+def admin_billing_subscription_trial(
+    db, other_merchant, another_platform, admin_billing_tiers
+):
+    """Create a trial subscription on another merchant/platform."""
+    now = datetime.now(UTC)
+    sub = MerchantSubscription(
+        merchant_id=other_merchant.id,
+        platform_id=another_platform.id,
+        tier_id=admin_billing_tiers[0].id,
+        status=SubscriptionStatus.TRIAL.value,
+        period_start=now,
+        period_end=now + timedelta(days=14),
+        trial_ends_at=now + timedelta(days=14),
+    )
+    db.add(sub)
+    db.commit()
+    db.refresh(sub)
+    return sub
+
+
+@pytest.fixture
+def admin_billing_multiple_subscriptions(
+    db, test_merchant, other_merchant, test_platform, another_platform,
+    admin_billing_tiers, platform_factory,
+):
+    """Create 3 subscriptions across different merchants/platforms."""
+    now = datetime.now(UTC)
+    third_platform = platform_factory()
+
+    subs = [
+        MerchantSubscription(
+            merchant_id=test_merchant.id,
+            platform_id=test_platform.id,
+            tier_id=admin_billing_tiers[0].id,
+            status=SubscriptionStatus.ACTIVE.value,
+            period_start=now,
+            period_end=now + timedelta(days=30),
+        ),
+        MerchantSubscription(
+            merchant_id=other_merchant.id,
+            platform_id=another_platform.id,
+            tier_id=admin_billing_tiers[1].id,
+            status=SubscriptionStatus.TRIAL.value,
+            period_start=now,
+            period_end=now + timedelta(days=14),
+        ),
+        MerchantSubscription(
+            merchant_id=test_merchant.id,
+            platform_id=third_platform.id,
+            tier_id=admin_billing_tiers[2].id,
+            status=SubscriptionStatus.ACTIVE.value,
+            period_start=now,
+            period_end=now + timedelta(days=30),
+        ),
+    ]
+    db.add_all(subs)
+    db.commit()
+    for s in subs:
+        db.refresh(s)
+    return subs
+
+
+@pytest.fixture
+def admin_billing_history_records(db, test_merchant):
+    """Create 3 billing history records for test_merchant."""
+    records = []
+    for i in range(3):
+        record = BillingHistory(
+            merchant_id=test_merchant.id,
+            stripe_invoice_id=f"in_admin_test_{i}",
+            invoice_number=f"ADM-{i:03d}",
+            invoice_date=datetime.now(UTC) - timedelta(days=i * 30),
+            subtotal_cents=4900,
+            tax_cents=0,
+            total_cents=4900,
+            amount_paid_cents=4900,
+            currency="EUR",
+            status="paid",
+        )
+        records.append(record)
+    db.add_all(records)
+    db.commit()
+    for r in records:
+        db.refresh(r)
+    return records
diff --git a/app/modules/billing/tests/unit/test_billing_service.py b/app/modules/billing/tests/unit/test_billing_service.py
index 5b9d8ad3..579bb9c8 100644
--- a/app/modules/billing/tests/unit/test_billing_service.py
+++ b/app/modules/billing/tests/unit/test_billing_service.py
@@ -1,12 +1,11 @@
-# tests/unit/services/test_billing_service.py
+# app/modules/billing/tests/unit/test_billing_service.py
 """Unit tests for BillingService."""
 
-from datetime import datetime, timezone
+from datetime import UTC, datetime, timedelta
 from unittest.mock import MagicMock, patch
 
 import pytest
 
-from app.modules.tenancy.exceptions import StoreNotFoundException
 from app.modules.billing.services.billing_service import (
     BillingService,
     NoActiveSubscriptionError,
@@ -25,26 +24,107 @@ from app.modules.billing.models import (
 )
 
 
+# ============================================================================
+# Tier Lookup
+# ============================================================================
+
+
 @pytest.mark.unit
 @pytest.mark.billing
 class TestBillingServiceTiers:
     """Test suite for BillingService tier operations."""
 
     def setup_method(self):
-        """Initialize service instance before each test."""
         self.service = BillingService()
 
+    def test_get_tier_by_code_found(self, db, bs_tier_essential):
+        """Returns the active tier."""
+        tier = self.service.get_tier_by_code(db, "essential")
+        assert tier.code == "essential"
+
     def test_get_tier_by_code_not_found(self, db):
-        """Test getting non-existent tier raises error."""
+        """Nonexistent tier raises TierNotFoundError."""
         with pytest.raises(TierNotFoundError) as exc_info:
             self.service.get_tier_by_code(db, "nonexistent")
-
         assert exc_info.value.tier_code == "nonexistent"
 
+    def test_get_tier_by_code_inactive_not_returned(self, db, bs_tier_essential):
+        """Inactive tier raises TierNotFoundError (only active tiers returned)."""
+        bs_tier_essential.is_active = False
+        db.flush()
+
+        with pytest.raises(TierNotFoundError):
+            self.service.get_tier_by_code(db, "essential")
 
 
-# TestBillingServiceCheckout removed — depends on refactored store_id-based API
-# TestBillingServicePortal removed — depends on refactored store_id-based API
+# ============================================================================
+# Available Tiers with Upgrade/Downgrade Flags
+# ============================================================================
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestBillingServiceAvailableTiers:
+    """Test suite for get_available_tiers (upgrade/downgrade detection)."""
+
+    def setup_method(self):
+        self.service = BillingService()
+
+    def test_get_available_tiers_returns_all(self, db, bs_tiers):
+        """Returns all active public tiers."""
+        tier_list, tier_order = self.service.get_available_tiers(db, None)
+        assert len(tier_list) == 3
+
+    def test_get_available_tiers_marks_current(self, db, bs_tiers):
+        """Current tier is marked with is_current=True."""
+        tier_list, _ = self.service.get_available_tiers(db, bs_tiers[1].id)
+
+        current = [t for t in tier_list if t["is_current"]]
+        assert len(current) == 1
+        assert current[0]["code"] == "professional"
+
+    def test_get_available_tiers_upgrade_flags(self, db, bs_tiers):
+        """Tiers with higher display_order have can_upgrade=True."""
+        tier_list, _ = self.service.get_available_tiers(db, bs_tiers[0].id)
+
+        essential = next(t for t in tier_list if t["code"] == "essential")
+        professional = next(t for t in tier_list if t["code"] == "professional")
+        business = next(t for t in tier_list if t["code"] == "business")
+
+        assert essential["is_current"] is True
+        assert essential["can_upgrade"] is False
+        assert professional["can_upgrade"] is True
+        assert business["can_upgrade"] is True
+
+    def test_get_available_tiers_downgrade_flags(self, db, bs_tiers):
+        """Tiers with lower display_order have can_downgrade=True."""
+        tier_list, _ = self.service.get_available_tiers(db, bs_tiers[2].id)
+
+        essential = next(t for t in tier_list if t["code"] == "essential")
+        professional = next(t for t in tier_list if t["code"] == "professional")
+        business = next(t for t in tier_list if t["code"] == "business")
+
+        assert essential["can_downgrade"] is True
+        assert professional["can_downgrade"] is True
+        assert business["is_current"] is True
+        assert business["can_downgrade"] is False
+
+    def test_get_available_tiers_no_current_tier(self, db, bs_tiers):
+        """When current_tier_id is None, no tier is marked current."""
+        tier_list, _ = self.service.get_available_tiers(db, None)
+        assert all(t["is_current"] is False for t in tier_list)
+
+    def test_get_available_tiers_returns_tier_order_map(self, db, bs_tiers):
+        """Returns tier_order map of code → display_order."""
+        _, tier_order = self.service.get_available_tiers(db, None)
+        assert tier_order["essential"] == 1
+        assert tier_order["professional"] == 2
+        assert tier_order["business"] == 3
+
+
+# ============================================================================
+# Invoices
+# ============================================================================
 
 
 @pytest.mark.unit
@@ -53,17 +133,42 @@ class TestBillingServiceInvoices:
     """Test suite for BillingService invoice operations."""
 
     def setup_method(self):
-        """Initialize service instance before each test."""
         self.service = BillingService()
 
-    def test_get_invoices_empty(self, db, test_store):
-        """Test getting invoices when none exist."""
-        invoices, total = self.service.get_invoices(db, test_store.id)
-
+    def test_get_invoices_empty(self, db, test_merchant):
+        """Returns empty list and zero total when no invoices exist."""
+        invoices, total = self.service.get_invoices(db, test_merchant.id)
         assert invoices == []
         assert total == 0
 
-    # test_get_invoices_with_data and test_get_invoices_pagination removed — fixture model mismatch after migration
+    def test_get_invoices_with_data(self, db, bs_billing_history):
+        """Returns invoices for the merchant."""
+        merchant_id = bs_billing_history[0].merchant_id
+        invoices, total = self.service.get_invoices(db, merchant_id)
+        assert total == 3
+        assert len(invoices) == 3
+
+    def test_get_invoices_pagination(self, db, bs_billing_history):
+        """Pagination limits and offsets results."""
+        merchant_id = bs_billing_history[0].merchant_id
+        invoices, total = self.service.get_invoices(db, merchant_id, skip=0, limit=2)
+        assert total == 3
+        assert len(invoices) == 2
+
+        invoices2, _ = self.service.get_invoices(db, merchant_id, skip=2, limit=2)
+        assert len(invoices2) == 1
+
+    def test_get_invoices_ordered_by_date_desc(self, db, bs_billing_history):
+        """Invoices are returned newest first."""
+        merchant_id = bs_billing_history[0].merchant_id
+        invoices, _ = self.service.get_invoices(db, merchant_id)
+        dates = [inv.invoice_date for inv in invoices]
+        assert dates == sorted(dates, reverse=True)
+
+
+# ============================================================================
+# Add-ons
+# ============================================================================
 
 
 @pytest.mark.unit
@@ -72,55 +177,442 @@ class TestBillingServiceAddons:
     """Test suite for BillingService addon operations."""
 
     def setup_method(self):
-        """Initialize service instance before each test."""
         self.service = BillingService()
 
     def test_get_available_addons_empty(self, db):
-        """Test getting addons when none exist."""
+        """Returns empty when no addons exist."""
         addons = self.service.get_available_addons(db)
         assert addons == []
 
     def test_get_available_addons_with_data(self, db, test_addon_products):
-        """Test getting all available addons."""
+        """Returns all active addons."""
         addons = self.service.get_available_addons(db)
-
         assert len(addons) == 3
         assert all(addon.is_active for addon in addons)
 
     def test_get_available_addons_by_category(self, db, test_addon_products):
-        """Test filtering addons by category."""
+        """Filters by category."""
         domain_addons = self.service.get_available_addons(db, category="domain")
-
         assert len(domain_addons) == 1
         assert domain_addons[0].category == "domain"
 
     def test_get_store_addons_empty(self, db, test_store):
-        """Test getting store addons when none purchased."""
+        """Returns empty when store has no purchased addons."""
         addons = self.service.get_store_addons(db, test_store.id)
         assert addons == []
 
 
-
-# TestBillingServiceCancellation removed — depends on refactored store_id-based API
-# TestBillingServiceStore removed — get_store method was removed from BillingService
+# ============================================================================
+# Subscription with Tier
+# ============================================================================
 
 
-# ==================== Fixtures ====================
+@pytest.mark.unit
+@pytest.mark.billing
+class TestBillingServiceSubscriptionWithTier:
+    """Tests for get_subscription_with_tier."""
+
+    def setup_method(self):
+        self.service = BillingService()
+
+    def test_get_subscription_with_tier_existing(
+        self, db, bs_subscription
+    ):
+        """Returns (subscription, tier) tuple for existing subscription."""
+        sub, tier = self.service.get_subscription_with_tier(
+            db, bs_subscription.merchant_id, bs_subscription.platform_id
+        )
+        assert sub.id == bs_subscription.id
+        assert tier is not None
+        assert tier.code == "essential"
+
+    def test_get_subscription_with_tier_creates_if_missing(
+        self, db, test_merchant, test_platform, bs_tier_essential
+    ):
+        """Creates a trial subscription when none exists (via get_or_create)."""
+        sub, tier = self.service.get_subscription_with_tier(
+            db, test_merchant.id, test_platform.id
+        )
+        assert sub is not None
+        assert sub.status == SubscriptionStatus.TRIAL.value
+
+
+# ============================================================================
+# Change Tier
+# ============================================================================
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestBillingServiceChangeTier:
+    """Tests for change_tier (the tier upgrade/downgrade flow)."""
+
+    def setup_method(self):
+        self.service = BillingService()
+
+    def test_change_tier_no_subscription_raises(self, db, bs_tiers):
+        """Raises NoActiveSubscriptionError when no subscription exists."""
+        with pytest.raises(NoActiveSubscriptionError):
+            self.service.change_tier(db, 99999, 99999, "professional", False)
+
+    def test_change_tier_no_stripe_subscription_raises(
+        self, db, bs_subscription, bs_tiers
+    ):
+        """Raises when subscription has no stripe_subscription_id."""
+        # bs_subscription has no Stripe IDs
+        with pytest.raises(NoActiveSubscriptionError):
+            self.service.change_tier(
+                db,
+                bs_subscription.merchant_id,
+                bs_subscription.platform_id,
+                "professional",
+                False,
+            )
+
+    def test_change_tier_nonexistent_tier_raises(
+        self, db, bs_stripe_subscription
+    ):
+        """Raises TierNotFoundError for nonexistent tier."""
+        with pytest.raises(TierNotFoundError):
+            self.service.change_tier(
+                db,
+                bs_stripe_subscription.merchant_id,
+                bs_stripe_subscription.platform_id,
+                "nonexistent",
+                False,
+            )
+
+    def test_change_tier_no_price_id_raises(
+        self, db, bs_stripe_subscription, bs_tiers
+    ):
+        """Raises StripePriceNotConfiguredError when tier has no Stripe price."""
+        # bs_tiers have no stripe_price_* set
+        with pytest.raises(StripePriceNotConfiguredError):
+            self.service.change_tier(
+                db,
+                bs_stripe_subscription.merchant_id,
+                bs_stripe_subscription.platform_id,
+                "professional",
+                False,
+            )
+
+    @patch("app.modules.billing.services.billing_service.stripe_service")
+    def test_change_tier_success(
+        self, mock_stripe, db, bs_stripe_subscription, bs_tiers_with_stripe
+    ):
+        """Successful tier change updates local subscription and calls Stripe."""
+        mock_stripe.is_configured = True
+        mock_stripe.update_subscription = MagicMock()
+
+        result = self.service.change_tier(
+            db,
+            bs_stripe_subscription.merchant_id,
+            bs_stripe_subscription.platform_id,
+            "professional",
+            False,
+        )
+
+        assert result["new_tier"] == "professional"
+        assert result["effective_immediately"] is True
+        assert bs_stripe_subscription.tier_id == bs_tiers_with_stripe[1].id
+        mock_stripe.update_subscription.assert_called_once()
+
+    @patch("app.modules.billing.services.billing_service.stripe_service")
+    def test_change_tier_annual_uses_annual_price(
+        self, mock_stripe, db, bs_stripe_subscription, bs_tiers_with_stripe
+    ):
+        """Annual billing selects stripe_price_annual_id."""
+        mock_stripe.is_configured = True
+        mock_stripe.update_subscription = MagicMock()
+
+        self.service.change_tier(
+            db,
+            bs_stripe_subscription.merchant_id,
+            bs_stripe_subscription.platform_id,
+            "professional",
+            True,
+        )
+
+        call_args = mock_stripe.update_subscription.call_args
+        assert call_args.kwargs["new_price_id"] == "price_pro_annual"
+
+
+# ============================================================================
+# _is_upgrade
+# ============================================================================
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestBillingServiceIsUpgrade:
+    """Tests for _is_upgrade helper."""
+
+    def setup_method(self):
+        self.service = BillingService()
+
+    def test_is_upgrade_true(self, db, bs_tiers):
+        """Higher display_order is an upgrade."""
+        assert self.service._is_upgrade(db, bs_tiers[0].id, bs_tiers[2].id) is True
+
+    def test_is_upgrade_false_downgrade(self, db, bs_tiers):
+        """Lower display_order is not an upgrade."""
+        assert self.service._is_upgrade(db, bs_tiers[2].id, bs_tiers[0].id) is False
+
+    def test_is_upgrade_same_tier(self, db, bs_tiers):
+        """Same tier is not an upgrade."""
+        assert self.service._is_upgrade(db, bs_tiers[1].id, bs_tiers[1].id) is False
+
+    def test_is_upgrade_none_ids(self, db):
+        """None tier IDs return False."""
+        assert self.service._is_upgrade(db, None, None) is False
+        assert self.service._is_upgrade(db, None, 1) is False
+        assert self.service._is_upgrade(db, 1, None) is False
+
+
+# ============================================================================
+# Cancel Subscription
+# ============================================================================
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestBillingServiceCancel:
+    """Tests for cancel_subscription."""
+
+    def setup_method(self):
+        self.service = BillingService()
+
+    def test_cancel_no_subscription_raises(self, db):
+        """Raises when no subscription found."""
+        with pytest.raises(NoActiveSubscriptionError):
+            self.service.cancel_subscription(db, 99999, 99999, None, False)
+
+    def test_cancel_no_stripe_id_raises(self, db, bs_subscription):
+        """Raises when subscription has no stripe_subscription_id."""
+        with pytest.raises(NoActiveSubscriptionError):
+            self.service.cancel_subscription(
+                db,
+                bs_subscription.merchant_id,
+                bs_subscription.platform_id,
+                "reason",
+                False,
+            )
+
+    @patch("app.modules.billing.services.billing_service.stripe_service")
+    def test_cancel_success(self, mock_stripe, db, bs_stripe_subscription):
+        """Cancellation records timestamp and reason."""
+        mock_stripe.is_configured = True
+        mock_stripe.cancel_subscription = MagicMock()
+
+        result = self.service.cancel_subscription(
+            db,
+            bs_stripe_subscription.merchant_id,
+            bs_stripe_subscription.platform_id,
+            "Too expensive",
+            False,
+        )
+
+        assert result["message"] == "Subscription cancelled successfully"
+        assert bs_stripe_subscription.cancelled_at is not None
+        assert bs_stripe_subscription.cancellation_reason == "Too expensive"
+        mock_stripe.cancel_subscription.assert_called_once()
+
+
+# ============================================================================
+# Reactivate Subscription
+# ============================================================================
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestBillingServiceReactivate:
+    """Tests for reactivate_subscription."""
+
+    def setup_method(self):
+        self.service = BillingService()
+
+    def test_reactivate_no_subscription_raises(self, db):
+        """Raises when no subscription found."""
+        with pytest.raises(NoActiveSubscriptionError):
+            self.service.reactivate_subscription(db, 99999, 99999)
+
+    def test_reactivate_not_cancelled_raises(self, db, bs_stripe_subscription):
+        """Raises SubscriptionNotCancelledError when not cancelled."""
+        with pytest.raises(SubscriptionNotCancelledError):
+            self.service.reactivate_subscription(
+                db,
+                bs_stripe_subscription.merchant_id,
+                bs_stripe_subscription.platform_id,
+            )
+
+    @patch("app.modules.billing.services.billing_service.stripe_service")
+    def test_reactivate_success(self, mock_stripe, db, bs_stripe_subscription):
+        """Reactivation clears cancellation and calls Stripe."""
+        mock_stripe.is_configured = True
+        mock_stripe.reactivate_subscription = MagicMock()
+
+        # Cancel first
+        bs_stripe_subscription.cancelled_at = datetime.now(UTC)
+        bs_stripe_subscription.cancellation_reason = "Testing"
+        db.flush()
+
+        result = self.service.reactivate_subscription(
+            db,
+            bs_stripe_subscription.merchant_id,
+            bs_stripe_subscription.platform_id,
+        )
+
+        assert result["message"] == "Subscription reactivated successfully"
+        assert bs_stripe_subscription.cancelled_at is None
+        assert bs_stripe_subscription.cancellation_reason is None
+        mock_stripe.reactivate_subscription.assert_called_once()
+
+
+# ============================================================================
+# Checkout Session
+# ============================================================================
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestBillingServiceCheckout:
+    """Tests for create_checkout_session."""
+
+    def setup_method(self):
+        self.service = BillingService()
+
+    def test_checkout_stripe_not_configured_raises(self, db, bs_tiers_with_stripe):
+        """Raises PaymentSystemNotConfiguredError when Stripe is off."""
+        with patch(
+            "app.modules.billing.services.billing_service.stripe_service"
+        ) as mock_stripe:
+            mock_stripe.is_configured = False
+
+            with pytest.raises(PaymentSystemNotConfiguredError):
+                self.service.create_checkout_session(
+                    db, 1, 1, "essential", False, "http://ok", "http://cancel"
+                )
+
+    def test_checkout_nonexistent_tier_raises(self, db):
+        """Raises TierNotFoundError for nonexistent tier."""
+        with patch(
+            "app.modules.billing.services.billing_service.stripe_service"
+        ) as mock_stripe:
+            mock_stripe.is_configured = True
+
+            with pytest.raises(TierNotFoundError):
+                self.service.create_checkout_session(
+                    db, 1, 1, "nonexistent", False, "http://ok", "http://cancel"
+                )
+
+
+# ============================================================================
+# Portal Session
+# ============================================================================
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestBillingServicePortal:
+    """Tests for create_portal_session."""
+
+    def setup_method(self):
+        self.service = BillingService()
+
+    def test_portal_stripe_not_configured_raises(self, db):
+        """Raises PaymentSystemNotConfiguredError when Stripe is off."""
+        with patch(
+            "app.modules.billing.services.billing_service.stripe_service"
+        ) as mock_stripe:
+            mock_stripe.is_configured = False
+
+            with pytest.raises(PaymentSystemNotConfiguredError):
+                self.service.create_portal_session(db, 1, 1, "http://return")
+
+    def test_portal_no_subscription_raises(self, db):
+        """Raises NoActiveSubscriptionError when no subscription found."""
+        with patch(
+            "app.modules.billing.services.billing_service.stripe_service"
+        ) as mock_stripe:
+            mock_stripe.is_configured = True
+
+            with pytest.raises(NoActiveSubscriptionError):
+                self.service.create_portal_session(db, 99999, 99999, "http://return")
+
+    def test_portal_no_customer_id_raises(self, db, bs_subscription):
+        """Raises when subscription has no stripe_customer_id."""
+        with patch(
+            "app.modules.billing.services.billing_service.stripe_service"
+        ) as mock_stripe:
+            mock_stripe.is_configured = True
+
+            with pytest.raises(NoActiveSubscriptionError):
+                self.service.create_portal_session(
+                    db,
+                    bs_subscription.merchant_id,
+                    bs_subscription.platform_id,
+                    "http://return",
+                )
+
+
+# ============================================================================
+# Upcoming Invoice
+# ============================================================================
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestBillingServiceUpcomingInvoice:
+    """Tests for get_upcoming_invoice."""
+
+    def setup_method(self):
+        self.service = BillingService()
+
+    def test_upcoming_invoice_no_subscription_raises(self, db):
+        """Raises when no subscription exists."""
+        with pytest.raises(NoActiveSubscriptionError):
+            self.service.get_upcoming_invoice(db, 99999, 99999)
+
+    def test_upcoming_invoice_no_customer_id_raises(self, db, bs_subscription):
+        """Raises when subscription has no stripe_customer_id."""
+        with pytest.raises(NoActiveSubscriptionError):
+            self.service.get_upcoming_invoice(
+                db, bs_subscription.merchant_id, bs_subscription.platform_id
+            )
+
+    def test_upcoming_invoice_stripe_not_configured_returns_empty(
+        self, db, bs_stripe_subscription
+    ):
+        """Returns empty invoice when Stripe is not configured."""
+        with patch(
+            "app.modules.billing.services.billing_service.stripe_service"
+        ) as mock_stripe:
+            mock_stripe.is_configured = False
+
+            result = self.service.get_upcoming_invoice(
+                db,
+                bs_stripe_subscription.merchant_id,
+                bs_stripe_subscription.platform_id,
+            )
+
+            assert result["amount_due_cents"] == 0
+            assert result["line_items"] == []
+
+
+# ============================================================================
+# Fixtures
+# ============================================================================
 
 
 @pytest.fixture
-def test_subscription_tier(db):
-    """Create a basic subscription tier."""
+def bs_tier_essential(db):
+    """Create essential subscription tier."""
     tier = SubscriptionTier(
         code="essential",
         name="Essential",
         description="Essential plan",
         price_monthly_cents=4900,
         price_annual_cents=49000,
-        orders_per_month=100,
-        products_limit=200,
-        team_members=1,
-        features=["basic_support"],
         display_order=1,
         is_active=True,
         is_public=True,
@@ -132,39 +624,14 @@ def test_subscription_tier(db):
 
 
 @pytest.fixture
-def test_subscription_tier_with_stripe(db):
-    """Create a subscription tier with Stripe configuration."""
-    tier = SubscriptionTier(
-        code="essential",
-        name="Essential",
-        description="Essential plan",
-        price_monthly_cents=4900,
-        price_annual_cents=49000,
-        orders_per_month=100,
-        products_limit=200,
-        team_members=1,
-        features=["basic_support"],
-        display_order=1,
-        is_active=True,
-        is_public=True,
-        stripe_product_id="prod_test123",
-        stripe_price_monthly_id="price_test123",
-        stripe_price_annual_id="price_test456",
-    )
-    db.add(tier)
-    db.commit()
-    db.refresh(tier)
-    return tier
-
-
-@pytest.fixture
-def test_subscription_tiers(db):
-    """Create multiple subscription tiers."""
+def bs_tiers(db):
+    """Create three tiers without Stripe config."""
     tiers = [
         SubscriptionTier(
             code="essential",
             name="Essential",
             price_monthly_cents=4900,
+            price_annual_cents=49000,
             display_order=1,
             is_active=True,
             is_public=True,
@@ -173,6 +640,7 @@ def test_subscription_tiers(db):
             code="professional",
             name="Professional",
             price_monthly_cents=9900,
+            price_annual_cents=99000,
             display_order=2,
             is_active=True,
             is_public=True,
@@ -181,6 +649,7 @@ def test_subscription_tiers(db):
             code="business",
             name="Business",
             price_monthly_cents=19900,
+            price_annual_cents=199000,
             display_order=3,
             is_active=True,
             is_public=True,
@@ -188,136 +657,107 @@ def test_subscription_tiers(db):
     ]
     db.add_all(tiers)
     db.commit()
-    for tier in tiers:
-        db.refresh(tier)
+    for t in tiers:
+        db.refresh(t)
     return tiers
 
 
 @pytest.fixture
-def test_subscription(db, test_store):
-    """Create a basic subscription for testing."""
-    # Create tier first
-    tier = SubscriptionTier(
-        code="essential",
-        name="Essential",
-        price_monthly_cents=4900,
-        display_order=1,
-        is_active=True,
-        is_public=True,
-    )
-    db.add(tier)
+def bs_tiers_with_stripe(db):
+    """Create tiers with Stripe price IDs configured."""
+    tiers = [
+        SubscriptionTier(
+            code="essential",
+            name="Essential",
+            price_monthly_cents=4900,
+            price_annual_cents=49000,
+            display_order=1,
+            is_active=True,
+            is_public=True,
+            stripe_product_id="prod_essential",
+            stripe_price_monthly_id="price_ess_monthly",
+            stripe_price_annual_id="price_ess_annual",
+        ),
+        SubscriptionTier(
+            code="professional",
+            name="Professional",
+            price_monthly_cents=9900,
+            price_annual_cents=99000,
+            display_order=2,
+            is_active=True,
+            is_public=True,
+            stripe_product_id="prod_professional",
+            stripe_price_monthly_id="price_pro_monthly",
+            stripe_price_annual_id="price_pro_annual",
+        ),
+        SubscriptionTier(
+            code="business",
+            name="Business",
+            price_monthly_cents=19900,
+            price_annual_cents=199000,
+            display_order=3,
+            is_active=True,
+            is_public=True,
+            stripe_product_id="prod_business",
+            stripe_price_monthly_id="price_biz_monthly",
+            stripe_price_annual_id="price_biz_annual",
+        ),
+    ]
+    db.add_all(tiers)
     db.commit()
-
-    subscription = MerchantSubscription(
-        store_id=test_store.id,
-        tier="essential",
-        status=SubscriptionStatus.ACTIVE,
-        period_start=datetime.now(timezone.utc),
-        period_end=datetime.now(timezone.utc),
-    )
-    db.add(subscription)
-    db.commit()
-    db.refresh(subscription)
-    return subscription
+    for t in tiers:
+        db.refresh(t)
+    return tiers
 
 
 @pytest.fixture
-def test_active_subscription(db, test_store):
+def bs_subscription(db, test_merchant, test_platform, bs_tier_essential):
+    """Create an active merchant subscription (no Stripe IDs)."""
+    now = datetime.now(UTC)
+    sub = MerchantSubscription(
+        merchant_id=test_merchant.id,
+        platform_id=test_platform.id,
+        tier_id=bs_tier_essential.id,
+        status=SubscriptionStatus.ACTIVE.value,
+        period_start=now,
+        period_end=now + timedelta(days=30),
+    )
+    db.add(sub)
+    db.commit()
+    db.refresh(sub)
+    return sub
+
+
+@pytest.fixture
+def bs_stripe_subscription(db, test_merchant, test_platform, bs_tier_essential):
     """Create an active subscription with Stripe IDs."""
-    # Create tier first if not exists
-    tier = db.query(SubscriptionTier).filter(SubscriptionTier.code == "essential").first()
-    if not tier:
-        tier = SubscriptionTier(
-            code="essential",
-            name="Essential",
-            price_monthly_cents=4900,
-            display_order=1,
-            is_active=True,
-            is_public=True,
-        )
-        db.add(tier)
-        db.commit()
-
-    subscription = MerchantSubscription(
-        store_id=test_store.id,
-        tier="essential",
-        status=SubscriptionStatus.ACTIVE,
+    now = datetime.now(UTC)
+    sub = MerchantSubscription(
+        merchant_id=test_merchant.id,
+        platform_id=test_platform.id,
+        tier_id=bs_tier_essential.id,
+        status=SubscriptionStatus.ACTIVE.value,
         stripe_customer_id="cus_test123",
         stripe_subscription_id="sub_test123",
-        period_start=datetime.now(timezone.utc),
-        period_end=datetime.now(timezone.utc),
+        period_start=now,
+        period_end=now + timedelta(days=30),
     )
-    db.add(subscription)
+    db.add(sub)
     db.commit()
-    db.refresh(subscription)
-    return subscription
+    db.refresh(sub)
+    return sub
 
 
 @pytest.fixture
-def test_cancelled_subscription(db, test_store):
-    """Create a cancelled subscription."""
-    # Create tier first if not exists
-    tier = db.query(SubscriptionTier).filter(SubscriptionTier.code == "essential").first()
-    if not tier:
-        tier = SubscriptionTier(
-            code="essential",
-            name="Essential",
-            price_monthly_cents=4900,
-            display_order=1,
-            is_active=True,
-            is_public=True,
-        )
-        db.add(tier)
-        db.commit()
-
-    subscription = MerchantSubscription(
-        store_id=test_store.id,
-        tier="essential",
-        status=SubscriptionStatus.ACTIVE,
-        stripe_customer_id="cus_test123",
-        stripe_subscription_id="sub_test123",
-        period_start=datetime.now(timezone.utc),
-        period_end=datetime.now(timezone.utc),
-        cancelled_at=datetime.now(timezone.utc),
-        cancellation_reason="Too expensive",
-    )
-    db.add(subscription)
-    db.commit()
-    db.refresh(subscription)
-    return subscription
-
-
-@pytest.fixture
-def test_billing_history(db, test_store):
-    """Create a billing history record."""
-    record = BillingHistory(
-        store_id=test_store.id,
-        stripe_invoice_id="in_test123",
-        invoice_number="INV-001",
-        invoice_date=datetime.now(timezone.utc),
-        subtotal_cents=4900,
-        tax_cents=0,
-        total_cents=4900,
-        amount_paid_cents=4900,
-        currency="EUR",
-        status="paid",
-    )
-    db.add(record)
-    db.commit()
-    db.refresh(record)
-    return record
-
-
-@pytest.fixture
-def test_multiple_invoices(db, test_store):
-    """Create multiple billing history records."""
+def bs_billing_history(db, test_merchant):
+    """Create billing history records for test_merchant."""
     records = []
-    for i in range(5):
+    for i in range(3):
         record = BillingHistory(
-            store_id=test_store.id,
-            stripe_invoice_id=f"in_test{i}",
-            invoice_number=f"INV-{i:03d}",
-            invoice_date=datetime.now(timezone.utc),
+            merchant_id=test_merchant.id,
+            stripe_invoice_id=f"in_bs_test_{i}",
+            invoice_number=f"BS-{i:03d}",
+            invoice_date=datetime.now(UTC) - timedelta(days=i * 30),
             subtotal_cents=4900,
             tax_cents=0,
             total_cents=4900,
@@ -328,6 +768,8 @@ def test_multiple_invoices(db, test_store):
         records.append(record)
     db.add_all(records)
     db.commit()
+    for r in records:
+        db.refresh(r)
     return records
 
 
diff --git a/app/modules/billing/tests/unit/test_subscription_service.py b/app/modules/billing/tests/unit/test_subscription_service.py
new file mode 100644
index 00000000..019275e8
--- /dev/null
+++ b/app/modules/billing/tests/unit/test_subscription_service.py
@@ -0,0 +1,579 @@
+# app/modules/billing/tests/unit/test_subscription_service.py
+"""Unit tests for SubscriptionService (merchant-level subscription management)."""
+
+from datetime import UTC, datetime, timedelta
+
+import pytest
+
+from app.modules.billing.exceptions import SubscriptionNotFoundException
+from app.modules.billing.models import (
+    MerchantSubscription,
+    SubscriptionStatus,
+    SubscriptionTier,
+    TierCode,
+)
+from app.modules.billing.services.subscription_service import SubscriptionService
+
+
+# ============================================================================
+# Tier Information
+# ============================================================================
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestSubscriptionServiceTierLookup:
+    """Tests for tier lookup methods."""
+
+    def setup_method(self):
+        self.service = SubscriptionService()
+
+    def test_get_tier_by_code_returns_tier(self, db, billing_tier_essential):
+        """Existing tier code returns the tier object."""
+        tier = self.service.get_tier_by_code(db, "essential")
+
+        assert tier is not None
+        assert tier.code == "essential"
+        assert tier.id == billing_tier_essential.id
+
+    def test_get_tier_by_code_nonexistent_returns_none(self, db):
+        """Nonexistent tier code returns None (does not raise)."""
+        tier = self.service.get_tier_by_code(db, "nonexistent")
+        assert tier is None
+
+    def test_get_tier_id_returns_id(self, db, billing_tier_essential):
+        """get_tier_id returns the integer ID for a valid code."""
+        tier_id = self.service.get_tier_id(db, "essential")
+        assert tier_id == billing_tier_essential.id
+
+    def test_get_tier_id_nonexistent_returns_none(self, db):
+        """get_tier_id returns None for nonexistent code."""
+        tier_id = self.service.get_tier_id(db, "nonexistent")
+        assert tier_id is None
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestSubscriptionServiceGetAllTiers:
+    """Tests for get_all_tiers with platform filtering."""
+
+    def setup_method(self):
+        self.service = SubscriptionService()
+
+    def test_get_all_tiers_returns_active_public_tiers(self, db, billing_tiers):
+        """Returns only active + public tiers, ordered by display_order."""
+        tiers = self.service.get_all_tiers(db)
+
+        assert len(tiers) == 3
+        assert tiers[0].code == "essential"
+        assert tiers[1].code == "professional"
+        assert tiers[2].code == "business"
+
+    def test_get_all_tiers_excludes_inactive(self, db, billing_tiers):
+        """Inactive tiers are excluded."""
+        # Deactivate one
+        billing_tiers[0].is_active = False
+        db.flush()
+
+        tiers = self.service.get_all_tiers(db)
+        assert len(tiers) == 2
+        assert all(t.code != "essential" for t in tiers)
+
+    def test_get_all_tiers_excludes_non_public(self, db, billing_tiers):
+        """Non-public tiers are excluded."""
+        billing_tiers[2].is_public = False
+        db.flush()
+
+        tiers = self.service.get_all_tiers(db)
+        assert len(tiers) == 2
+        assert all(t.code != "business" for t in tiers)
+
+    def test_get_all_tiers_with_platform_filter(self, db, test_platform, billing_tiers):
+        """Platform filter returns platform-specific tiers + global tiers."""
+        # Make one tier platform-specific
+        billing_tiers[0].platform_id = test_platform.id
+        # billing_tiers[1] and [2] remain global (platform_id=NULL)
+        db.flush()
+
+        tiers = self.service.get_all_tiers(db, platform_id=test_platform.id)
+
+        codes = {t.code for t in tiers}
+        assert "essential" in codes  # Platform-specific
+        assert "professional" in codes  # Global
+        assert "business" in codes  # Global
+
+    def test_get_all_tiers_platform_filter_excludes_other_platform(
+        self, db, test_platform, another_platform, billing_tiers
+    ):
+        """Tiers belonging to another platform are excluded."""
+        billing_tiers[0].platform_id = another_platform.id
+        db.flush()
+
+        tiers = self.service.get_all_tiers(db, platform_id=test_platform.id)
+
+        codes = {t.code for t in tiers}
+        assert "essential" not in codes
+        assert len(tiers) == 2
+
+
+# ============================================================================
+# Merchant Subscription CRUD
+# ============================================================================
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestSubscriptionServiceGetMerchantSubscription:
+    """Tests for subscription retrieval methods."""
+
+    def setup_method(self):
+        self.service = SubscriptionService()
+
+    def test_get_merchant_subscription_found(
+        self, db, billing_subscription
+    ):
+        """Returns subscription when it exists."""
+        sub = self.service.get_merchant_subscription(
+            db,
+            billing_subscription.merchant_id,
+            billing_subscription.platform_id,
+        )
+        assert sub is not None
+        assert sub.id == billing_subscription.id
+
+    def test_get_merchant_subscription_not_found(self, db):
+        """Returns None when no subscription exists."""
+        sub = self.service.get_merchant_subscription(db, 99999, 99999)
+        assert sub is None
+
+    def test_get_merchant_subscription_eager_loads_tier(
+        self, db, billing_subscription
+    ):
+        """Subscription comes with tier relationship loaded."""
+        sub = self.service.get_merchant_subscription(
+            db,
+            billing_subscription.merchant_id,
+            billing_subscription.platform_id,
+        )
+        # Accessing .tier should not trigger a lazy load error
+        assert sub.tier is not None
+        assert sub.tier.code == "essential"
+
+    def test_get_merchant_subscription_eager_loads_platform(
+        self, db, billing_subscription
+    ):
+        """Subscription comes with platform relationship loaded."""
+        sub = self.service.get_merchant_subscription(
+            db,
+            billing_subscription.merchant_id,
+            billing_subscription.platform_id,
+        )
+        assert sub.platform is not None
+
+    def test_get_merchant_subscriptions_all(
+        self, db, test_merchant, test_platform, another_platform, billing_tier_essential
+    ):
+        """Returns all subscriptions for a merchant across platforms."""
+        now = datetime.now(UTC)
+        sub1 = MerchantSubscription(
+            merchant_id=test_merchant.id,
+            platform_id=test_platform.id,
+            tier_id=billing_tier_essential.id,
+            status=SubscriptionStatus.ACTIVE.value,
+            period_start=now,
+            period_end=now + timedelta(days=30),
+        )
+        sub2 = MerchantSubscription(
+            merchant_id=test_merchant.id,
+            platform_id=another_platform.id,
+            tier_id=billing_tier_essential.id,
+            status=SubscriptionStatus.TRIAL.value,
+            period_start=now,
+            period_end=now + timedelta(days=14),
+        )
+        db.add_all([sub1, sub2])
+        db.flush()
+
+        subs = self.service.get_merchant_subscriptions(db, test_merchant.id)
+        assert len(subs) == 2
+
+    def test_get_merchant_subscriptions_empty(self, db, test_merchant):
+        """Returns empty list when merchant has no subscriptions."""
+        subs = self.service.get_merchant_subscriptions(db, test_merchant.id)
+        assert subs == []
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestSubscriptionServiceGetOrRaise:
+    """Tests for get_subscription_or_raise."""
+
+    def setup_method(self):
+        self.service = SubscriptionService()
+
+    def test_get_subscription_or_raise_found(self, db, billing_subscription):
+        """Returns subscription when found."""
+        sub = self.service.get_subscription_or_raise(
+            db,
+            billing_subscription.merchant_id,
+            billing_subscription.platform_id,
+        )
+        assert sub.id == billing_subscription.id
+
+    def test_get_subscription_or_raise_not_found(self, db):
+        """Raises SubscriptionNotFoundException when not found."""
+        with pytest.raises(SubscriptionNotFoundException):
+            self.service.get_subscription_or_raise(db, 99999, 99999)
+
+
+# ============================================================================
+# Create Subscription
+# ============================================================================
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestSubscriptionServiceCreate:
+    """Tests for create_merchant_subscription."""
+
+    def setup_method(self):
+        self.service = SubscriptionService()
+
+    def test_create_trial_subscription(
+        self, db, test_merchant, test_platform, billing_tier_essential
+    ):
+        """Default creation starts a 14-day trial."""
+        sub = self.service.create_merchant_subscription(
+            db, test_merchant.id, test_platform.id
+        )
+
+        assert sub.merchant_id == test_merchant.id
+        assert sub.platform_id == test_platform.id
+        assert sub.status == SubscriptionStatus.TRIAL.value
+        assert sub.tier_id == billing_tier_essential.id
+        assert sub.trial_ends_at is not None
+        assert sub.is_annual is False
+
+    def test_create_subscription_period_end_matches_trial(
+        self, db, test_merchant, test_platform, billing_tier_essential
+    ):
+        """Period end is set to trial_days from now."""
+        sub = self.service.create_merchant_subscription(
+            db, test_merchant.id, test_platform.id, trial_days=7
+        )
+
+        expected_end = sub.period_start + timedelta(days=7)
+        # Allow 1 second tolerance
+        assert abs((sub.period_end - expected_end).total_seconds()) < 1
+
+    def test_create_annual_subscription(
+        self, db, test_merchant, test_platform, billing_tier_essential
+    ):
+        """Annual subscription with no trial gets 365-day period."""
+        sub = self.service.create_merchant_subscription(
+            db, test_merchant.id, test_platform.id, trial_days=0, is_annual=True
+        )
+
+        assert sub.status == SubscriptionStatus.ACTIVE.value
+        assert sub.is_annual is True
+        assert sub.trial_ends_at is None
+        expected_end = sub.period_start + timedelta(days=365)
+        assert abs((sub.period_end - expected_end).total_seconds()) < 1
+
+    def test_create_monthly_subscription(
+        self, db, test_merchant, test_platform, billing_tier_essential
+    ):
+        """Monthly subscription with no trial gets 30-day period."""
+        sub = self.service.create_merchant_subscription(
+            db, test_merchant.id, test_platform.id, trial_days=0, is_annual=False
+        )
+
+        assert sub.status == SubscriptionStatus.ACTIVE.value
+        assert sub.is_annual is False
+        assert sub.trial_ends_at is None
+        expected_end = sub.period_start + timedelta(days=30)
+        assert abs((sub.period_end - expected_end).total_seconds()) < 1
+
+    def test_create_subscription_custom_tier(
+        self, db, test_merchant, test_platform, billing_tiers
+    ):
+        """Can specify a custom tier code."""
+        sub = self.service.create_merchant_subscription(
+            db, test_merchant.id, test_platform.id, tier_code="professional"
+        )
+        assert sub.tier_id == billing_tiers[1].id
+
+    def test_create_duplicate_raises(
+        self, db, billing_subscription
+    ):
+        """Creating a second subscription for same merchant+platform raises ValueError."""
+        with pytest.raises(ValueError, match="already has a subscription"):
+            self.service.create_merchant_subscription(
+                db,
+                billing_subscription.merchant_id,
+                billing_subscription.platform_id,
+            )
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestSubscriptionServiceGetOrCreate:
+    """Tests for get_or_create_subscription."""
+
+    def setup_method(self):
+        self.service = SubscriptionService()
+
+    def test_get_or_create_returns_existing(self, db, billing_subscription):
+        """Returns existing subscription without creating a new one."""
+        sub = self.service.get_or_create_subscription(
+            db,
+            billing_subscription.merchant_id,
+            billing_subscription.platform_id,
+        )
+        assert sub.id == billing_subscription.id
+
+    def test_get_or_create_creates_new(
+        self, db, test_merchant, test_platform, billing_tier_essential
+    ):
+        """Creates new subscription when none exists."""
+        sub = self.service.get_or_create_subscription(
+            db, test_merchant.id, test_platform.id
+        )
+        assert sub is not None
+        assert sub.merchant_id == test_merchant.id
+        assert sub.status == SubscriptionStatus.TRIAL.value
+
+
+# ============================================================================
+# Upgrade / Cancel / Reactivate
+# ============================================================================
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestSubscriptionServiceUpgradeTier:
+    """Tests for upgrade_tier."""
+
+    def setup_method(self):
+        self.service = SubscriptionService()
+
+    def test_upgrade_tier_changes_tier_id(self, db, billing_subscription, billing_tiers):
+        """Tier is updated to the new tier's ID."""
+        sub = self.service.upgrade_tier(
+            db,
+            billing_subscription.merchant_id,
+            billing_subscription.platform_id,
+            "professional",
+        )
+        assert sub.tier_id == billing_tiers[1].id
+
+    def test_upgrade_from_trial_activates(self, db, billing_subscription, billing_tiers):
+        """Upgrading from trial status sets status to active."""
+        billing_subscription.status = SubscriptionStatus.TRIAL.value
+        db.flush()
+
+        sub = self.service.upgrade_tier(
+            db,
+            billing_subscription.merchant_id,
+            billing_subscription.platform_id,
+            "professional",
+        )
+        assert sub.status == SubscriptionStatus.ACTIVE.value
+
+    def test_upgrade_active_stays_active(self, db, billing_subscription, billing_tiers):
+        """Upgrading an already-active subscription keeps status active."""
+        billing_subscription.status = SubscriptionStatus.ACTIVE.value
+        db.flush()
+
+        sub = self.service.upgrade_tier(
+            db,
+            billing_subscription.merchant_id,
+            billing_subscription.platform_id,
+            "professional",
+        )
+        assert sub.status == SubscriptionStatus.ACTIVE.value
+
+    def test_upgrade_nonexistent_tier_raises(self, db, billing_subscription):
+        """Upgrading to a nonexistent tier raises ValueError."""
+        with pytest.raises(ValueError, match="not found"):
+            self.service.upgrade_tier(
+                db,
+                billing_subscription.merchant_id,
+                billing_subscription.platform_id,
+                "nonexistent_tier",
+            )
+
+    def test_upgrade_no_subscription_raises(self, db):
+        """Upgrading when no subscription exists raises SubscriptionNotFoundException."""
+        with pytest.raises(SubscriptionNotFoundException):
+            self.service.upgrade_tier(db, 99999, 99999, "professional")
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestSubscriptionServiceCancel:
+    """Tests for cancel_subscription."""
+
+    def setup_method(self):
+        self.service = SubscriptionService()
+
+    def test_cancel_sets_status(self, db, billing_subscription):
+        """Cancellation sets status to cancelled."""
+        sub = self.service.cancel_subscription(
+            db,
+            billing_subscription.merchant_id,
+            billing_subscription.platform_id,
+        )
+        assert sub.status == SubscriptionStatus.CANCELLED.value
+
+    def test_cancel_records_timestamp(self, db, billing_subscription):
+        """Cancellation records cancelled_at."""
+        sub = self.service.cancel_subscription(
+            db,
+            billing_subscription.merchant_id,
+            billing_subscription.platform_id,
+        )
+        assert sub.cancelled_at is not None
+
+    def test_cancel_with_reason(self, db, billing_subscription):
+        """Cancellation stores the reason."""
+        sub = self.service.cancel_subscription(
+            db,
+            billing_subscription.merchant_id,
+            billing_subscription.platform_id,
+            reason="Too expensive",
+        )
+        assert sub.cancellation_reason == "Too expensive"
+
+    def test_cancel_no_subscription_raises(self, db):
+        """Cancelling when no subscription exists raises."""
+        with pytest.raises(SubscriptionNotFoundException):
+            self.service.cancel_subscription(db, 99999, 99999)
+
+
+@pytest.mark.unit
+@pytest.mark.billing
+class TestSubscriptionServiceReactivate:
+    """Tests for reactivate_subscription."""
+
+    def setup_method(self):
+        self.service = SubscriptionService()
+
+    def test_reactivate_sets_active(self, db, billing_subscription):
+        """Reactivation sets status back to active."""
+        # First cancel
+        self.service.cancel_subscription(
+            db,
+            billing_subscription.merchant_id,
+            billing_subscription.platform_id,
+        )
+
+        sub = self.service.reactivate_subscription(
+            db,
+            billing_subscription.merchant_id,
+            billing_subscription.platform_id,
+        )
+        assert sub.status == SubscriptionStatus.ACTIVE.value
+
+    def test_reactivate_clears_cancellation(self, db, billing_subscription):
+        """Reactivation clears cancelled_at and cancellation_reason."""
+        self.service.cancel_subscription(
+            db,
+            billing_subscription.merchant_id,
+            billing_subscription.platform_id,
+            reason="Testing",
+        )
+
+        sub = self.service.reactivate_subscription(
+            db,
+            billing_subscription.merchant_id,
+            billing_subscription.platform_id,
+        )
+        assert sub.cancelled_at is None
+        assert sub.cancellation_reason is None
+
+    def test_reactivate_no_subscription_raises(self, db):
+        """Reactivating when no subscription exists raises."""
+        with pytest.raises(SubscriptionNotFoundException):
+            self.service.reactivate_subscription(db, 99999, 99999)
+
+
+# ============================================================================
+# Fixtures
+# ============================================================================
+
+
+@pytest.fixture
+def billing_tier_essential(db):
+    """Create essential subscription tier."""
+    tier = SubscriptionTier(
+        code="essential",
+        name="Essential",
+        description="Essential plan",
+        price_monthly_cents=4900,
+        price_annual_cents=49000,
+        display_order=1,
+        is_active=True,
+        is_public=True,
+    )
+    db.add(tier)
+    db.commit()
+    db.refresh(tier)
+    return tier
+
+
+@pytest.fixture
+def billing_tiers(db):
+    """Create essential, professional, and business tiers."""
+    tiers = [
+        SubscriptionTier(
+            code="essential",
+            name="Essential",
+            price_monthly_cents=4900,
+            price_annual_cents=49000,
+            display_order=1,
+            is_active=True,
+            is_public=True,
+        ),
+        SubscriptionTier(
+            code="professional",
+            name="Professional",
+            price_monthly_cents=9900,
+            price_annual_cents=99000,
+            display_order=2,
+            is_active=True,
+            is_public=True,
+        ),
+        SubscriptionTier(
+            code="business",
+            name="Business",
+            price_monthly_cents=19900,
+            price_annual_cents=199000,
+            display_order=3,
+            is_active=True,
+            is_public=True,
+        ),
+    ]
+    db.add_all(tiers)
+    db.commit()
+    for t in tiers:
+        db.refresh(t)
+    return tiers
+
+
+@pytest.fixture
+def billing_subscription(db, test_merchant, test_platform, billing_tier_essential):
+    """Create an active merchant subscription (essential tier)."""
+    now = datetime.now(UTC)
+    sub = MerchantSubscription(
+        merchant_id=test_merchant.id,
+        platform_id=test_platform.id,
+        tier_id=billing_tier_essential.id,
+        status=SubscriptionStatus.ACTIVE.value,
+        period_start=now,
+        period_end=now + timedelta(days=30),
+    )
+    db.add(sub)
+    db.commit()
+    db.refresh(sub)
+    return sub