OptionaldiscountOptionalquantityHow quantity is sent: 'expand' (default) adds one item per unit, with ids
A-1, A-2, ...; 'multiply' adds one item priced at unit price × quantity.
OptionalshippingShipping fee, added as the last item. A zero fee adds no item.
Discount on the items (not on shipping): a fixed amount such as
'13.48', or{ percent: 10 }. It is spread over the items in proportion to their totals.