Set up Shippo return label using the 'is_return' flag

Steps

1. Create a Shippo shipment with the 'is_return' field set to true in the shipment POST body; swap the 'address_from' and 'address_to' compared to the outbound shipment so the customer address is the origin.
2. Include 'extra' object with any return-specific options such as signature confirmation; not all options available on outbound labels are supported on return labels.
3. Retrieve rates from the returned 'rates' array and select an appropriate service; return labels often have specific carrier programs (e.g., USPS Return Service) that appear in the rates.
4. Purchase a transaction using the chosen rate's 'object_id'; the resulting label 'label_url' can be emailed to the customer or included in the package.
5. Store the return transaction's 'tracking_number' to monitor when the package is dropped off and in transit.