Def.
Preferred Carrier allow merchant to define a specific Carrier and its allied service to be made available to shopper while they request for Return or Exchange. With single Carrier providing multiple services at varied cost, merchant can define the shopper experience and can select a default carrier to be given to Shoppers, giving a standardised experience throughout.
How to enable Preferred Carrier
Toggle the switch next to preferred carrier to activate the dropdowns below. Enabling this should allow you to select Carrier among the ones you have already configured.
Select the carrier from the activated list of your Aggregator account
Choose the SLA/Service from the dropdown you would want to be given as preferred services to your shoppers.
Save the changes from the bottom right corner to enable the services.