Hmmmm in my opinion there is nothing to guarantee once they leave your table they will really provide a good home to a betta fish. You can always ask about any previous experience in fish keeping.
If I was in your shoes and I was going to charge a fee I would first explain why I am charging (i.e. housing, medication, etc.) to cover my expenses. Then I would ask for a 7-10 dollar donation. I think a true pet lover wont be discouraged by the amount. At the very least you have educated someone on the level of care needed for betta's.
RIP Blue Jay 9.15.12
1 Veil tail Betta, 1 Albino Betta, 1 Fan Tail Betta, 3 Cory Catfish