If you're integrating a gift card API for the first time, the vendor demo will look great. The sandbox will work on the first try. You'll think this is going to be a clean two-week sprint. Then you ship to production and discover the same three problems every team hits: Duplicate orders because retries weren't idempotent. Missed transactions because webhooks didn't arrive. A finance team that can't reconcile your numbers against the vendor's numbers. I write a lot about gift card infrastructure (full disclosure: I work on a product in the space). The piece below is the developer-shaped version of a longer evaluation guide we published — focused on what actually breaks at runtime and what to ask vendors before you commit. Idempotency, or you'll double-charge your customers If your POST /orders call times out, you don't know whether the order went through. So you retry. Without an idempotency key, you've now ordered the card twice and charged the customer twice. Every serious vendor supports idempotency keys.…