Use hosted checkout or approved provider SDKs; create sessions server-side with trusted price IDs and customer context.

How to apply it

Verify signatures, deduplicate event IDs, process state transitions idempotently, and reconcile internal state with provider records. Events may be duplicated, delayed, or out of order. Never store raw card data.

Worked example

After checkout redirect, show pending status until verified provider state confirms the subscription.

Common mistakes to avoid

Granting paid access from an unverified browser redirect or blindly overwriting state from an old event.

Practice task

Implement this concept in the capstone. Add one positive test, one negative or failure-path test, and a short note explaining the trade-off you chose.