Plan the payment flow
Decide what happens before payment, during checkout, after success and after failure. The application should have a clear internal order state rather than assuming every browser response means payment succeeded.
Verify transactions server-side
Payment providers expose mechanisms for verification and webhooks. Use the provider’s current documentation and credentials, and never expose secret keys in frontend code.
Design for real customers
Explain what the customer is buying, show the total clearly and provide a useful confirmation path. Mobile checkout should be tested carefully because many customers will use phones.
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