In my opinion, you should be paid full for the header by the group. This way if SSAC mysteriously makes your header disappear your covered. I would not send a thing until you receive at least 90% of the funding.
The other item is, if SSAC can't make it, the group would be refunded any money put in. I think this would be more than fair to everyone as long as you get the header back.
I would also suggest that as people donate, you ask for the 3-5% for the paypal fees.