Implement ACP checkout sessions as a merchant endpoint so AI agents like ChatGPT can complete purchases programmatically

Verified steps

1. Register as an ACP merchant via the OpenAI Commerce Developer portal and obtain your merchant credentials; review the spec version (latest stable: 2026-04-17) at agenticcommerce.dev
2. Implement the five required HTTPS+JSON endpoints: POST /checkout_sessions (create), POST /checkout_sessions/{id} (update), GET /checkout_sessions/{id} (read), POST /checkout_sessions/{id}/complete, and POST /checkout_sessions/{id}/cancel
3. On session create, return a full authoritative cart state including line items, taxes, shipping options, and available payment handlers; each update call must return the complete refreshed cart
4. Register a webhook URL to receive order lifecycle events (order.created, order.updated, etc.) so ChatGPT stays synchronized with fulfillment-grade order state
5. Test end-to-end in the ACP sandbox using the reference client from the agentic-commerce-protocol GitHub organization before going live
6. Monitor post-launch for spec version changes, since ACP has shipped multiple breaking releases (2025-09-29 through 2026-04-17) and the maintainers (OpenAI and Stripe) publish migration guides with each version