Add card payments to an existing Next.js shop

We run a Next.js 15 storefront with a catalogue and a basket. We need card payments added: a checkout page, webhook handling, order statuses in the admin, and refunds. What exists: the repository, Figma designs, a provider sandbox account. What we need: working payments in production, tests covering the payment flow, and a short runbook for our team. Done means a test order completes the full cycle — payment, webhook, status, refund.