It is possible to enter a Purchase Order Number at the time of purchase that will be referenced on your invoice. Once you have signed up for an Enterprise Account, you will be able to enter PO numbers or Project Notes at the time of purchase.  These notes will be included on the emailed invoice that you receive after purchase.  When you reach the payment screen (where you either choose to pay with your Corporate Advantage Account or enter credit card information), you will see a white box labeled "Your Note".  You can enter any information here that you think you may want in the future to remember the conditions of this order.