My company sells custom designed items. No two are the same, and the prices are different. The client emails a description of what he wants, we create the item and send him a photo of it. If he likes it, I enter the item description and price on the Paypal site. Paypal then creates an invoice and emails it to him. He pays it, Paypal notifies me, I ship the item to the client. Is there a way to modify the Paypal invoice, so the customer can enter a coupon code and get a discount?