Transient Yamato-side failures (carrier system temporarily down or unreachable) on the Create Shipment endpoint (POST /v1/shipments) now return HTTP 503 instead of 400.
This lets API clients treat them as temporary and retry safely, rather than reading a 400 as a permanent input error.
The error body (code and message) is unchanged — only the HTTP status code changed.
If your integration branches on status codes, note this class of error moved from 400 to 503. The new response is documented in the API reference (developer.shipandco.com).
