I think Preston's right in separating the sitting fee, from which you derive the donation, from the product prices. Whether the right answer is $25, $15 or some other amount depends on where you are, how many people you expect to get, etc. But the sitting fee and some reference to where to view and order the prints/merchandise seems to me to be the right answer. I try not to include anything in the sitting fee except the sitting. Product is always separate.

Good luck,

Jim