Ecommerce Integration Review

Ecommerce Integration Review

Purpose

Review the correctness, security, and resilience of ecommerce integration code: product catalog sync, cart and checkout logic, payment provider integration (Stripe, PayPal, Adyen), webhook handling, inventory sync, tax/shipping calculation, and order lifecycle management.

When to use

• Reviewing Stripe, PayPal, Adyen, or similar payment gateway integration code.
• Auditing webhook handlers that receive payment, order, or fulfillment events from external platforms.
• Reviewing catalog sync pipelines between a PIM, ERP, or marketplace and a storefront.
• Evaluating checkout flow code for race conditions, double-charge risks, or inventory oversell.
• Reviewing tax engine integration (Avalara, TaxJar) or shipping rate API calls.
• Auditing integration between Shopify, WooCommerce, BigCommerce, or headless commerce and third-party services.

When not to use

• Pure UI/UX review of a product listing page with no integration logic — use a front-end review.
• Database schema design with no commerce business logic.
• Infrastructure (CDN, caching layer) unless it directly affects checkout availability.

Procedure

1. Map the integration boundaries

• Identify all external systems: payment provider, shipping carrier API, tax engine, ERP/warehouse.
• Document each integration point: direction (inbound/outbound), protocol (REST/webhook/GraphQL/EDI), authentication method (API key, OAuth, HMAC signature).
• Check that secrets are stored in environment variables — never hardcoded.

2. Payment integration

• Stripe: verify PaymentIntent creation with correct amount (in smallest currency unit), currency, and idempotency_key.
• Confirm payment_intent.succeeded webhook is used to fulfill orders — not the client-side redirect alone.
• PayPal: verify IPN or Webhook signature validation before order fulfillment.
• Check that partial captures and refunds are handled; confirm amount_to_capture does not exceed authorized amount.
• Ensure test mode / sandbox credentials are never deployed to production (detect by key prefix: sk_test_, sandbox endpoints).

3. Idempotency and duplicate prevention

• Every payment initiation must use an idempotency key tied to the order ID, not a random UUID generated per retry.
• Webhook handlers must be idempotent: processing the same event twice must not create duplicate orders or double-charge.
• Implement event deduplication using the provider's event ID stored in a processed-events table or cache (TTL ≥ 24 h).

4. Inventory and stock sync

• Check for race conditions: concurrent checkouts for the same SKU must use a database-level lock or a reservation pattern (reserve → purchase → confirm/release).
• Catalog sync must handle partial failures: if 500 of 1,000 SKUs sync, the remaining 500 must retry, not silently drop.
• Stock level writes must be atomic — use transactions or compare-and-swap (UPDATE stock SET qty = qty - 1 WHERE qty > 0 AND sku = ?).
• Oversell prevention: enforce qty >= 0 constraint at the DB level, not only in application code.

5. Tax and shipping

• Tax calculations must be performed server-side at checkout — never trust a client-supplied tax amount.
• Verify tax engine is called with accurate ship_from, ship_to, and line-item details.
• Shipping rate API calls must be cached per quote session, not re-fetched per page load.
• Confirm that free-shipping thresholds and discount stacking are evaluated after discounts are applied to the subtotal.

6. Order lifecycle

• Order state machine: define allowed transitions (pending → processing → shipped → delivered → refunded). Block invalid transitions.
• Fulfillment webhooks (shipment created, delivered) must update order status and notify the customer.
• Refund flow: verify refund amount ≤ captured amount; reversal must mark order line items as refunded in the local DB.
• Cancellation: release reserved inventory before cancelling payment.

7. PCI and data handling

• Card data must never touch application servers — use client-side tokenization (Stripe Elements, Braintree Drop-in).
• No raw PAN, CVV, or full track data in logs, analytics payloads, or error messages.
• Webhook payloads containing card info (rare but seen in older integrations) must be immediately truncated before storage.
• Confirm the integration is scoped to SAQ A or SAQ A-EP — if the server handles raw card data, escalate as critical.

8. Error handling and retries

• Retry transient failures (network timeout, 5xx from payment provider) with exponential back-off and jitter.
• Do not retry on 4xx (bad request, invalid card) — surface the error to the user.
• Failed webhook deliveries: implement a dead-letter queue and alerting after N retries.
• Log every payment event with order ID, provider event ID, amount, currency, and outcome — redact card details.

Checklist

Payment:

• Idempotency key used for payment initiation, tied to order/session ID.
• Server-side webhook signature verified before fulfillment.
• Test/sandbox credentials absent from production config.
• Partial capture and refund paths handled.
• Double-charge prevention: webhook deduplication implemented.

Inventory:

• Stock reservation or DB-level locking prevents oversell.
• Atomic stock decrement at checkout confirmation.
• Catalog sync handles partial failure with retry.

Compliance:

• No raw card data on application servers.
• No PAN/CVV in logs or analytics.
• Tax calculated server-side, not from client payload.

Resilience:

• Retry with back-off for transient provider errors.
• Dead-letter queue for failed webhooks.
• Order state machine rejects invalid transitions.

Common issues & anti-patterns

• Fulfilling on client redirect: relying on the browser returning to a success URL to mark an order paid. Network failures prevent this; always use server-side webhooks.
• Random idempotency key per retry: generates duplicate charges on retry. Use a deterministic key: order_id + attempt_number.
• No webhook signature check: any external caller can spoof a payment_intent.succeeded event and get free orders.
• Oversell via concurrent requests: two simultaneous checkouts both read qty = 1, both decrement — oversell by 1. Fix with SELECT FOR UPDATE or optimistic locking.
• Tax from client payload: a modified frontend POST can set tax to $0. Always recalculate on the server.
• Logging req.body for payment webhooks: inadvertently logs raw card data or sensitive bank details. Log only the event ID and type.
• Synchronous inventory update during payment: if the payment step is slow and the inventory write times out, the transaction is left in a limbo state. Use eventual consistency with a saga pattern.

Required output

Return a structured report with:

• Summary: pass / conditional pass / blocked, and the primary risk category.
• Evidence: file + line references for each finding.
• Findings table: severity (critical / high / medium / low / info), category, description, remediation.
• PCI note: explicit statement on card data scope and SAQ level.
• Resilience assessment: idempotency, retry logic, and webhook deduplication status.
• Next handoff: staging test plan with specific scenarios (concurrent checkout, webhook replay, refund flow).

Safety

• Never print live API keys, Stripe secret keys, or payment provider webhooks secrets.
• Do not execute payment API calls or trigger test charges during review.
• Flag any PAN, CVV, or bank account data found in code or config as critical.
• If a vulnerability enables free-order fraud or double-charge, mark critical and block deployment.