Assign a source of truth for every field
Decide where products, prices, promotions, customers, tax, inventory, orders, fulfillment, cancellations, refunds, and payment status are mastered. Bidirectional synchronization without ownership rules creates loops and conflicting data.
Design for failure as a normal state
Use idempotent operations, queues, retries, rate-limit handling, clear error logs, and reconciliation reports. Test partial fulfillment, overselling, duplicate webhooks, changed orders, and refunds before launch.