- Allow the list of line items to be a non-associative array
where each item is identified by an "id" field
- Same for taxes inside line items, specify amount to refund in
a "refund_total" key as in the case of line items
- Allow "quantity" keys as synonyms of "qty"
Also calculate "amount" automatically if missing and when all the
line items and taxes have a valid "refund_total" key.