Shipping Rates – Return backup rates for 3xx and 4xx carrier responses

Carriers occasionally respond with HTTP 3xx redirects or 4xx client errors. Previously, this left merchants without shipping options at checkout. The rate service now extends its existing error-handling path to treat these responses as recoverable failures. This change triggers a fallback to backup rates instead of resulting in a hard failure. As a result, shoppers still see viable shipping methods, safeguarding conversion and improving overall checkout reliability.