Just keep in mind that the large print they bought is fine and good and, in fact, they liked it so much they came back for more. But the web site is a different matter now. This isn't an all-you-can-eat buffet where they can get all the BBQ they want and then go back for more fried chicken. I would not give them a thing. License the image, price it fairly and then send then point thm to you licensing.pixels.com site.
*If* you do give it to them, still have a signed agreement as to how the image can be used.