What Photo Printing/Selling Site Will Let me Do This?

I would like to offer to do family portraits for military veterans at print cost but allow for those who want to and are able to, to add more to the total bill for prints at check out..

Is there any photo hosting/printing site that allows you to add a box at checkout where a customer can add to the final total . . . thus, you could price the photos at cost for the customer to add to the cart but prompt the customer to add to the price whatever they would like to give in appreciation.