Crypto payments

A card charge either succeeds or fails, and the amount is the amount. A crypto payment is a request that someone may partially satisfy, over-satisfy, satisfy late, or satisfy after the price moved. Almost every defect in a crypto checkout comes from writing card-shaped code and meeting one of those cases in production.

This skill is provider-neutral. Concrete request/response shapes for one gateway live in references/heleket-provider.md; the invariants below hold for Coinbase Commerce, NOWPayments, BTCPay, Heleket and anything else that issues an invoice and calls you back. One thing is NOT provider-neutral and is labeled where it appears: the webhook signature algorithm, which differs per gateway.