How to Build Reliable SMS Order Receipt Delivery with Idempotency and Retry Logic

Sending order receipts via SMS requires treating timeouts as unknown outcomes rather than failures, since a timed-out request may have already been accepted by the transport layer. Developers are advised to persist an immutable receipt intent — including order ID, payment-settlement event ID, template version, and a single idempotency key — before dispatching any message. A narrow state machine should govern delivery status, distinguishing between pending, submitting, accepted, delivered, and permanent failure, rather than relying on a generic retry wrapper. Every retry must reuse the same idempotency key to prevent a single settled payment from generating duplicate receipts. Template versions should also be pinned and immutable at the time of intent creation, ensuring that a retry hours later does not render a materially different receipt due to content or data changes.