If you frequently use the same return service and package type for your returns, you can set it as a default in your Shipping settings.
NOTE: This option is for return labels created after the initial shipment, and will not apply for return labels created to accompany the outgoing label.
Click Settings (the Wrench icon in the upper right).
Now, click Returns.
If your default is to use the same service/package type that was used for the outgoing shipment, you can use this default setting.
You can also choose any other services and package types available in your account.
Once set, the default service will be auto-selected in the Returns window on any order for which you create a return label.
To see how to create a return label you can send with the outgoing shipment, check out this article!
To see how to enable the Branded Returns Portal that allows your customers to create their own return labels, check out this article!