Calculate the suggested refund for an order or return. Supports two workflows: (1) direct refund from an order with specified line items, or (2) refund after a return where line items and totals are pre-calculated from the return.

Required fields:

• order_id or return_id (one must be provided)

A successful request returns the suggested refund details for the specified order or return.

Returns with exchange items (return_id only):

• Send the exchange items in exchange_line_items, together with the return line items in refund_line_items, to see what processing them together
  would do. Their prices are netted, and financial_outcome tells whether money is refunded (REFUND), the customer owes money (INVOICE), or nothing
  is owed either way (EVEN).
• financial_outcome is for the whole order: the items are first netted against what the order already owes. The restocking fees and the return
  shipping fees are already taken out of it.
• Without exchange_line_items, the amounts are calculated as for a return without exchange items, and return_context.exchange_line_items still
  lists the exchange items left to process.
• The display data of the exchange items (title, variant_title, sku, image_url, unit_price) comes from the synced product catalog.