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skills/ck-payment-integration/SKILL.md

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+---
+name: ck-payment-integration
+description: Integrate payments with SePay (VietQR), Polar, Stripe, Paddle, or Creem.io. Use for checkout flows, subscription management, webhook handling, QR code payments, and software licensing.
+---
+
+# Payment Integration
+
+Production payment processing across SePay, Polar, Stripe, Paddle, and Creem.io.
+
+## When to Use
+
+- Setting up payment gateway checkout or processing
+- Building subscription management (trials, upgrades, billing)
+- Handling webhooks with idempotency
+- Implementing QR code payments (VietQR/NAPAS)
+- Software licensing with device activation
+- Multi-provider order management
+
+## Don't Use When
+
+- Simple donation buttons (use Ko-fi or Buy Me a Coffee embed)
+- Internal transfer tools without customer-facing payments
+
+## Platform Selection
+
+| Platform | Best For |
+|----------|----------|
+| **SePay** | Vietnamese market, VND, bank transfers, VietQR |
+| **Polar** | Global SaaS, subscriptions, automated benefits |
+| **Stripe** | Enterprise, Connect platforms, custom checkout |
+| **Paddle** | MoR subscriptions, global tax compliance |
+| **Creem.io** | MoR + licensing, revenue splits, no-code checkout |
+
+## General Integration Flow
+
+```
+auth → products → checkout → webhooks → events
+```
+
+## Key Capabilities
+
+| Platform | Rate Limit | Highlights |
+|----------|-----------|------------|
+| SePay | 2 req/s | QR/bank/cards, 44+ VN banks |
+| Polar | 300 req/min | MoR, usage billing, GitHub/Discord benefits |
+| Stripe | varies | CheckoutSessions, Billing, Connect, Payment Element |
+| Paddle | varies | MoR overlay/inline, Retain churn prevention, tax |
+| Creem.io | varies | MoR, device licensing, revenue splits |
+
+## Security Rules
+
+- Store all API keys in environment variables — never hardcode
+- Always verify webhook signatures before processing events
+- Use idempotency keys for checkout and subscription operations
+- Implement proper session token handling for embedded flows
+
+## Reference Files
+
+- `references/sepay.md` - SePay auth, VietQR, webhooks, SDK
+- `references/polar.md` - Polar products, checkouts, subscriptions, benefits
+- `references/stripe.md` - Stripe best practices, SDKs, CLI, Payment Element
+- `references/paddle.md` - Paddle MoR, overlay/inline checkout, subscriptions
+- `references/creem.md` - Creem MoR, licensing, no-code checkout, webhooks
+- `references/multi-provider.md` - Unified orders, currency conversion patterns
+
+## External Docs
+
+- Stripe: https://docs.stripe.com/llms.txt
+- Paddle: https://developer.paddle.com/llms.txt
+- Creem: https://docs.creem.io/llms.txt