Yeah, I was thinking about that too. One thought I had (which others have suggested) is to encourage people to donate in small amounts— rather than drop $25 on the card if you're feeling generous, you should drop $5 on it each morning for a week.

But since that's kind of a hassle, then you'd whip up a quick app that lets people easily amortize a larger contribution over a series of coffee-scale contributions. Connect that to their twitter account, and you can credit each individual coffee donation to an individual and report them individually.

It's effectively the same as having a larger account balance, but since you're not holding the money it feels more engaging and social for folks.