Shopify and Amazon use different identifiers for the same shipping carrier. If a Shopify carrier name doesn't match an Amazon carrier code, the shipment sync fails. Map them once under Map Carrier Codes so every shipment pushes to Amazon with the right carrier identifier.
Steps
- Go to Settings → Order Settings → Shipment.
- Find Map Carrier Codes and toggle it on.
- Pick a value from the Shopify carrier code dropdown.
- Pick the matching value from the Amazon carrier code dropdown.
- Click Add New Carrier Code to create more mappings if you ship via multiple carriers.
- Save.