# Subscription & Billing System

The platform provides a comprehensive subscription and billing system for managing vendor subscriptions, usage limits, and payments through Stripe.

## Overview

The billing system enables:

- **Subscription Tiers**: Database-driven tier definitions with configurable limits
- **Usage Tracking**: Orders, products, and team member limits per tier
- **Stripe Integration**: Checkout sessions, customer portal, and webhook handling
- **Self-Service Billing**: Vendor-facing billing page for subscription management
- **Add-ons**: Optional purchasable items (domains, SSL, email packages)

## Architecture

### Database Models

All subscription models are defined in `models/database/subscription.py`:

| Model | Purpose |
|-------|---------|
| `SubscriptionTier` | Tier definitions with limits and Stripe price IDs |
| `VendorSubscription` | Per-vendor subscription status and usage |
| `AddOnProduct` | Purchasable add-ons (domains, SSL, email) |
| `VendorAddOn` | Add-ons purchased by each vendor |
| `StripeWebhookEvent` | Idempotency tracking for webhooks |
| `BillingHistory` | Invoice and payment history |

### Services

| Service | Location | Purpose |
|---------|----------|---------|
| `BillingService` | `app/services/billing_service.py` | Subscription operations, checkout, portal |
| `SubscriptionService` | `app/services/subscription_service.py` | Limit checks, usage tracking |
| `StripeService` | `app/services/stripe_service.py` | Core Stripe API operations |
| `StripeWebhookHandler` | `app/services/stripe_webhook_handler.py` | Webhook event processing |

### API Endpoints

All billing endpoints are under `/api/v1/vendor/billing`:

| Endpoint | Method | Purpose |
|----------|--------|---------|
| `/billing/subscription` | GET | Current subscription status & usage |
| `/billing/tiers` | GET | Available tiers for upgrade |
| `/billing/checkout` | POST | Create Stripe checkout session |
| `/billing/portal` | POST | Create Stripe customer portal session |
| `/billing/invoices` | GET | Invoice history |
| `/billing/addons` | GET | Available add-on products |
| `/billing/my-addons` | GET | Vendor's purchased add-ons |
| `/billing/cancel` | POST | Cancel subscription |
| `/billing/reactivate` | POST | Reactivate cancelled subscription |

## Subscription Tiers

### Default Tiers

| Tier | Price | Products | Orders/mo | Team |
|------|-------|----------|-----------|------|
| Essential | €49/mo | 200 | 100 | 1 |
| Professional | €99/mo | Unlimited | 500 | 3 |
| Business | €199/mo | Unlimited | 2000 | 10 |
| Enterprise | Custom | Unlimited | Unlimited | Unlimited |

### Tier Features

Each tier includes specific features stored in the `features` JSON column:

```python
tier.features = [
    "basic_support",       # Essential
    "priority_support",    # Professional+
    "analytics",           # Business+
    "api_access",          # Business+
    "white_label",         # Enterprise
    "custom_integrations", # Enterprise
]
```

## Limit Enforcement

Limits are enforced at the service layer:

### Orders
```python
# app/services/order_service.py
subscription_service.check_order_limit(db, vendor_id)
```

### Products
```python
# app/api/v1/vendor/products.py
subscription_service.check_product_limit(db, vendor_id)
```

### Team Members
```python
# app/services/vendor_team_service.py
subscription_service.can_add_team_member(db, vendor_id)
```

## Stripe Integration

### Configuration

Required environment variables:

```bash
STRIPE_SECRET_KEY=sk_test_...
STRIPE_PUBLISHABLE_KEY=pk_test_...
STRIPE_WEBHOOK_SECRET=whsec_...
STRIPE_TRIAL_DAYS=14  # Optional, default trial period
```

### Webhook Events

The system handles these Stripe events:

| Event | Handler |
|-------|---------|
| `checkout.session.completed` | Activates subscription, links customer |
| `customer.subscription.updated` | Updates tier, status, period |
| `customer.subscription.deleted` | Marks subscription cancelled |
| `invoice.paid` | Records payment, resets counters |
| `invoice.payment_failed` | Marks past due, increments retry count |

### Webhook Endpoint

Webhooks are received at `/api/v1/webhooks/stripe`:

```python
# Uses signature verification for security
event = stripe_service.construct_event(payload, stripe_signature)
```

## Vendor Billing Page

The vendor billing page is at `/vendor/{vendor_code}/billing`:

### Page Sections

1. **Current Plan**: Tier name, status, next billing date
2. **Usage Meters**: Products, orders, team members with limits
3. **Change Plan**: Upgrade/downgrade options
4. **Payment Method**: Link to Stripe portal
5. **Invoice History**: Recent invoices with PDF links

### JavaScript Component

The billing page uses Alpine.js (`static/vendor/js/billing.js`):

```javascript
function billingData() {
    return {
        subscription: null,
        tiers: [],
        invoices: [],

        async init() {
            await this.loadData();
        },

        async selectTier(tier) {
            const response = await this.apiPost('/billing/checkout', {
                tier_code: tier.code,
                is_annual: false
            });
            window.location.href = response.checkout_url;
        },

        async openPortal() {
            const response = await this.apiPost('/billing/portal', {});
            window.location.href = response.portal_url;
        }
    };
}
```

## Add-ons

### Available Add-ons

| Code | Name | Category | Price |
|------|------|----------|-------|
| `domain` | Custom Domain | domain | €15/year |
| `ssl_premium` | Premium SSL | ssl | €49/year |
| `email_5` | 5 Email Addresses | email | €5/month |
| `email_10` | 10 Email Addresses | email | €9/month |
| `email_25` | 25 Email Addresses | email | €19/month |

### Purchase Flow

1. Vendor selects add-on on billing page
2. For domains: enter domain name, validate availability
3. Create Stripe checkout session with add-on price
4. On webhook success: create `VendorAddOn` record

## Exception Handling

Custom exceptions for billing operations (`app/exceptions/billing.py`):

| Exception | HTTP Status | Description |
|-----------|-------------|-------------|
| `PaymentSystemNotConfiguredException` | 503 | Stripe not configured |
| `TierNotFoundException` | 404 | Invalid tier code |
| `StripePriceNotConfiguredException` | 400 | No Stripe price for tier |
| `NoActiveSubscriptionException` | 400 | Operation requires subscription |
| `SubscriptionNotCancelledException` | 400 | Cannot reactivate active subscription |

## Testing

Unit tests for the billing system:

```bash
# Run billing service tests
pytest tests/unit/services/test_billing_service.py -v

# Run webhook handler tests
pytest tests/unit/services/test_stripe_webhook_handler.py -v
```

### Test Coverage

- `BillingService`: Subscription queries, checkout, portal, cancellation
- `StripeWebhookHandler`: Event idempotency, checkout completion, invoice handling

## Migration

### Creating Tiers

Tiers are seeded via migration:

```python
# alembic/versions/xxx_add_subscription_billing_tables.py
def seed_subscription_tiers(op):
    op.bulk_insert(subscription_tiers_table, [
        {
            "code": "essential",
            "name": "Essential",
            "price_monthly_cents": 4900,
            "orders_per_month": 100,
            "products_limit": 200,
            "team_members": 1,
        },
        # ... more tiers
    ])
```

### Setting Up Stripe

1. Create products and prices in Stripe Dashboard
2. Update `SubscriptionTier` records with Stripe IDs:

```python
tier.stripe_product_id = "prod_xxx"
tier.stripe_price_monthly_id = "price_xxx"
tier.stripe_price_annual_id = "price_yyy"
```

3. Configure webhook endpoint in Stripe Dashboard:
   - URL: `https://yourdomain.com/api/v1/webhooks/stripe`
   - Events: `checkout.session.completed`, `customer.subscription.*`, `invoice.*`

## Security Considerations

- Webhook signatures verified before processing
- Idempotency keys prevent duplicate event processing
- Customer portal links are session-based and expire
- Stripe API key stored securely in environment variables