Idempotency
Send a unique Idempotency-Key (any UUID you generate) on every POST that creates,
charges, or sends:
Idempotency-Key: 1f2e3d4c-5b6a-7980-1234-56789abcdef0
- The first request with a given key is processed and its result cached for 24 hours.
- Any retry with the same key + same payload returns the original result (same status, same body) without repeating the side effect.
- The same key with a different payload returns
409 conflict.
This makes network retries safe — you'll never double-create an invoice or send a message twice.