The discount. Can be an item- or invoice-level discount, or both. Can be applied as a percent or amount. If you provide both amount and percent, amount takes precedent.
invoice_discountobject<discount>Optionalpercentstring<percentage>Optionalamountobject<money>Optionalcurrency_codestring<currency_code>RequiredvaluestringRequireditem_discountobject<money>Optionalcurrency_codestring<currency_code>RequiredvaluestringRequired