handle-checkout-flow

Use idempotency keys on payment intent creation so a repeated submission returns the original result instead of creating a second charge. The checkout submission must be idempotent end to end, including order persistence.

Validate coupons server-side only, checking code existence, validity window, usage limits, minimum order value, and applicable SKUs. Wrap validation and usage increment in an atomic transaction with a row-level lock (SELECT ... FOR UPDATE) to prevent TOCTOU race conditions.

Stripe Tax suits startups and mid-market merchants already in the Stripe ecosystem. TaxJar fits multi-channel merchants with moderate volumes, while Avalara AvaTax targets enterprises with high volume, complex Nexus rules, and custom ERP systems.

Use a two-phase atomic hold: soft reserve with a TTL at checkout submission, then hard commit on payment success or release on failure. Implement it with atomic SQL or Redis Lua rather than naive read-then-write sequences.

PCI DSS v4.1.0 requirement 6.4.3 mandates inventorying all payment page scripts and loading them with Subresource Integrity hashes and a strict CSP nonce. Requirement 11.6.1 adds tamper detection monitoring of payment page headers and scripts at least weekly.

Address validation is a mandatory isolated step that must occur before tax calculation or shipping carrier requests. Integrate APIs like Google Maps Address Validation, Loqate, or Smarty Streets to standardize raw input into carrier-compliant addresses.