That was an interesting project I did within my Facebook friends.
The deal was: I do their portrait and in exchange, the sitter would buy one of my children book from Amazon and give it as a present to any child. It could be any book. The books were quite cheap (around 4-12 euros).
Most of them bought one book after receiving their portrait…a few of them none. Two of the friends who didn’t buy any book asked me for another portrait for another family member, which I did for one and I didn’t for the second.