KickstarterForum.org

Has anyone done this? Like, your backer contributes $35 and you give them a code which gives them $50 off a future purchase?

I'm wondering about this because it would be impossible for me to pre-calculate the shipping for my every international country (I have a fairly heavy invention), and I don't think I want to (or can?) exclude donors outside the U.S. from contributing... Plus, I want people to have more time to consider which of the accessories they may want as well!

Thanks!