Payment Integration Plan

Plan a payment integration for {{use_case}} using {{provider}}. Cover the flow from checkout to confirmation, why to never handle raw card data yourself (use the providers secure elements), handling webhooks reliably and idempotently for the source of truth, the states a payment moves through, failures, retries, refunds, and reconciliation, security and PCI considerations, and testing with sandbox. Note the common mistakes (trusting the client, missing webhook handling, race conditions) that cause lost or double charges. Keep it secure and reliable.