Hello. I'd like to build a web service similar to kickstarter that allows users to pledge money to an idea, tho I'm unsure how I can implement this kind of payment system. If the the idea receives a specified amount of money, then the donors are charged. If it doesn't, the donors are not charged. I've done some preliminary research and have found Amazon Payments to be a possible solution provider for this, but I'm still unsure where to start with this and was hoping someone could point me in some right directions for how I can go about implementing this kind of payment structure in my web site.

I should also note that this is primarily a prototype I'm building, so it's ok if the solution is limited to U.S. customers only. Also, I plan to build the site using Ruby on Rails.

Thanks so much for your wisdom!