Mobile Payment Integration

You are a mobile payments expert. Help me integrate payment processing in my app.

Requirements:
- Platform: {{platform}}
- Payment Types: {{payment_types}} (cards, Apple Pay, Google Pay, in-app)
- Payment Provider: {{provider}} (Stripe, Braintree, Square, etc.)
- Subscription Support: {{subscriptions_needed}}
- Markets: {{target_markets}}

Please provide:

1. Apple Pay Integration:
   - Merchant ID setup
   - PKPaymentRequest configuration
   - Supported networks
   - Payment sheet presentation
   - Payment authorization handling
   - Backend token processing

2. Google Pay Integration:
   - Google Pay API setup
   - IsReadyToPayRequest
   - PaymentDataRequest
   - Payment method tokenization
   - Backend processing

3. Stripe SDK Integration:
   - SDK initialization
   - Payment Intent creation
   - Payment Sheet
   - Card element integration
   - 3D Secure handling
   - Error handling

4. In-App Purchases:
   - StoreKit 2 (iOS) / Google Play Billing
   - Product configuration
   - Purchase flow
   - Receipt validation
   - Subscription management
   - Restore purchases

5. Security:
   - PCI compliance considerations
   - Tokenization
   - Secure transmission
   - Fraud prevention

6. Backend Integration:
   - Webhook handling
   - Order management
   - Refund processing
   - Receipt validation server

7. Testing:
   - Sandbox environments
   - Test cards
   - Subscription testing
   - Edge case testing

8. Edge Cases:
   - Failed payments
   - Interrupted purchases
   - Subscription renewal failures
   - Cross-platform purchases