How can I update the Shipping, Tax, or Insurance Information in an Order or Transaction before the buyer checks out?
If you need to update the ShippingServiceCost, SalesTax, PaymentMethods or other checkout information in an order or a single item transaction, you can use the SendInvoice call or the AddOrder Call.
Use AddOrder to change information within the Order. A new Order will be generated, and the older Order will automatically become Inactive. No invoice will be sent to the Buyer
Use SendInvoice to send the new Order information to your Buyer. Only specify the OrderID and any Checkout Instructions. All Shipping, Tax and Insurance information will be taken from the Order.
Use SendInvoice to update the transaction, and send an email to the buyer. This is the only way to update information in the transaction. Specify any changes to Shipping, Tax or Insurance information in the SendInvoice call itself.