Process a return and create a refund for the associated order. This endpoint finalizes the return flow by issuing a refund tied to an existing return.

Required fields:

• order_id
• return_id

A successful request that refunds money returns 201 with the created refund details for the specified order. A return processed with
exchange items where nothing is refunded returns 200 with process_return_response instead.

Returns with exchange items:

• Send the exchange items in exchange_line_items, together with the return line items in refund_line_items, to process them in one request. Their
  prices are netted, and what happens to the money is Shopify's suggested outcome for the whole order, the same as financial_outcome of the
  suggested refund: the items are first netted against what the order already owes.
• When money is refunded (REFUND), the response is 201 with the refund, return_id, return_status, financial_outcome and the processed
  exchange_line_items.
• When the customer owes money (INVOICE) or nothing is owed either way (EVEN), nothing is refunded and the response is 200 with
  process_return_response. For an INVOICE with notify set to true, Shopify's invoice for the order is emailed to the customer.
• Shopify closes the return by itself a moment after everything in it is processed, so return_status can still read OPEN.
• Without exchange_line_items, the return line items are refunded as before. A return that still has exchange items stays open, and they are
  listed in pending_exchange_line_items. Processing them in a later request does not net them with the refunded items: the exchange items are then
  charged in full.
• Requests for Shopify are rejected with 400 when the return is not open, when a quantity is more than is left to process, or when transactions
  are sent and no refund is due. Custom integrations receive the request as is.