Quote - I may be able to donated up to $100 in gift certificates from time to time.

If buying these at any denomination counts towards the monthly goal of at least $100 for the next month rebate, a "collection plate" for low dollar amounts could work as a type of prize, with, say $100 total as a prize but consisting of x certificates from y people.  The odds are good the "plate" will hold more than needed, but those could be set aside for the next challenge.  Possibly a bit to manage, but it is an option.