1. How do you know which documents to buy with the donated credits?
2. How do you get access to the user's account? Do you store their password somewhere?
Maybe a simple trick would be to throw a little alert with the extension around the end of the quarter just to say, "Hey, it's the end of the quarter. Have you spent your entire free budget?"
Hm. There's an opportunity here, but we've never quite nailed it down.