Almost no fintech team builds card processing from scratch. Instead, they connect to a payment service provider such as Stripe or Adyen, which handles the card networks, fraud checks, and much of the compliance work. Your job is to integrate it correctly, so that every payment is recorded once, every failure is handled, and your own database always agrees with the provider.
You'll start with the payment intent flow: create the payment on your server, collect card details securely on the client, and confirm it with the provider, including the extra authentication step (3D Secure) required in many regions. Then you'll learn webhooks, the callbacks a provider sends to tell your server that a payment succeeded or failed, and why you must verify their signatures. Finally, you'll see how subscriptions and recurring billing depend on those same webhooks.
Keep in mind that webhooks can arrive late, twice, or out of order, so your handlers need to be safe to run more than once. Refunds and payouts follow the same pattern, and the provider's documentation covers them in detail. Once you've finished this module, move on to Banking APIs & Open Banking.
A fast overview of the Stripe payment flow: creating a payment intent on the server, collecting card details with Stripe.js, confirming the payment, and listening for events with webhooks.
A hands-on build of one-time payments with Stripe Checkout and with the Payment Intents API plus Elements, including 3D Secure, and a webhook endpoint that verifies Stripe's signature before fulfilling an order.
How subscriptions work end to end: creating a Stripe customer, starting a plan, and using webhooks for recurring payments to mark an account active or past due. This video is from 2017 and uses an older Stripe checkout flow, so focus on the concepts rather than the exact code.