Check out PayPal's new adaptive payments (or other similar services). I agree with byoung2 (though I don't have expertise in this area) -- you can receive a preapproval to charge funds once you have the amount needed to purchase through Amazon.

You'll probably need to charge a service fee, since the amount you'll get from your payment processor will be assessed their service fees.

With all the buzz around X.commerce, I'd definitely check them out -- all sorts of promise for easy group payments coming from them in the last few months.