They get nothing. I think those “reward” levels are just a cute way to trigger anchoring (http://en.wikipedia.org/wiki/Anchoring) and make the job of choosing how much money to give easier by suggesting some choices. In fact, the $13 contributors don’t get anything extra either – they get “access to Mailpile's online source code”, but that’s already available at https://github.com/pagekite/Mailpile.