If you have a common return service and package type that is used for your return shipments, this can be set up in your Shipping settings.
(Keep in mind that this will only work for return labels created after the initial shipment. This won't apply for outbound return labels)
Head to your Account Settings (the gear 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.
You can also automatically mark returns as "Received" if tracked.