You could also look at Amazon Flexible Payments, it looks like it was designed for the sort of use-case you are talking about. I would like to hear of people's

You could also look at Amazon Flexible Payments, it looks like it was designed for the sort of use-case you are talking about. I would like to hear of people's experiences with it though. It's coefficient of friction looks to be roughly similar to Paypal's and it's API seems reasonable. I've looked at it quite closely in the past.