Dee Dee,
I think 10% is more than enough. As for the folders, I agree that delivering the 4X6 in a Holiday folder on the spot and getting paid on the spot is the right answer. I would suggest, however, that you have a "Special" for Xmas cards with the image that they purchase. You can order those through almost any good print house (I use MPIX). You wouldn't have to add it to your website. Just print up a tent card and have the offer with you onsite.

Jim