Returns are not integrated with EasyPost out of the box

[aldesantis]

Currently, this extension provides a thin layer on top of Spree::ReturnAuthorization that allows users to easily generate a return label.

However, unlike for shipments, there is no automated integration between Solidus' out-of-the-box RMA system and EasyPost:

1. The EasyPost shipment ID related to the RMA is not persisted on the return authorization. Instead, a new EasyPost shipment is generated every time SolidusEasypost::ReturnAuthorization is instantiated.
2. There is no way to buy a rate for the return authorization from the UI.

Unfortunately, this is because Solidus, by default, has no concept of shipping rates for return authorizations. We could hack a solution together, but this is probably better fixed in the core.

[stale]

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