The item tax for each unit. If tax is specified,
purchase_units[].amount.breakdown.tax_total is required. Must equal tax *
quantity for all items. tax.value can not be a negative number.
The item price or rate per unit. If you specify unit_amount,
purchase_units[].amount.breakdown.item_total is required. Must equal
unit_amount * quantity for all items. unit_amount.value can not be a negative number.