If you use GoDataFeed API for Order Sync, then you can send a specific carrier value for orders that overrides the default carrier in the source settings. To do this, simply provide DestinationShippingCarrier.

This is the carrier value as defined in the Update Order Status request set by the seller or their system. If source settings option "Use Destination Carrier" is enabled, then this field overrides the default carrier from the source settings shipping methods mappings in the GoDatafeed app.

Values for DestinationShippingCarrier must be an accepted value from the order source otherwise the update will be rejected by the marketplace.

The following is from Walmart API reference article: Ship Order Lines - Walmart Developer Portal (walmart.com)

Accepted Carrier values from Walmart

The package shipment carrier.

Valid entries are:

• UPS
• USPS
• FedEx
• Airborne
• OnTrac
• DHL
• LS (LaserShip)
• UDS (United Delivery Service)
• UPSMI (UPS Mail Innovations)
• FDX
• PILOT
• ESTES
• SAIA
• FEDEXSP (FedEx SmartPost)