have you got a picture of the shirt that will be available? that might encourage people from further away to support the cause especially if a cool design, I know I will.
also maybe sneak in a custom flipper bat key ring etc for another level - enough to cover the cost but also help those from further away to get something unique as most of the rewards are for people that live in the area. I can design it for free and send you the file to get the parts from shapeways.
and you are not offering shirt to be shipped outside the US, maybe just allow a extra small reasonable fee to cover overseas.