1. You set the payment amount, but the default is exponential. (Edit: didn't read your comment thoroughly enough the first time to know you were referring to the OP, not the general policy.)
2. Payments are done via credit card, although Paypal support seems to be in the works.
3. It does not appear to be a non-profit. It looks like a business where Beeminder's living is made off of people paying when they fail their goals.