BTW, both Visa and Mastercard have a rule that there are no "minimum charge" requirements. ie, those stores/restaurants (why is it always an asian owned store?) that have the "minimum charge $5.00" are bogus. It may have changed, I heard this a few years ago, but I doubt it. HOWEVER I'm sure in a donation situation there is nothing that requires someone to take your offering.
NOW... if they have it all set up to automatically send the charge on to your issuer, then maybe we have somehing here...