Why a carrier integration that passes in sandbox can still fail on the first real shipment.
Carrier sandboxes often drift from production. Credentials, data validation, enabled products, account entitlements, and even the freshness of the API contract can differ. A green sandbox test proves your code can speak to one environment, not that the production account is ready.
Compare environments with explicit evidence: correlation IDs, captured requests, response detail, and contract versions. Runbooks should assume drift is possible until production behavior has been observed directly.