The other day, Tim Bray pointed to eSellerate, a company that provides software download fulfillment services. Sounds like Tim had a good experience as a purchaser using their service. The cost is 10-15% of your revenue, which is pretty steep when compared to doing your own credit card processing (2-4% + some fixed fees, depending on which cards you accept, etc.). However, eSellerate handles everything for you and it sounds like they do it well. Seems like a nice thing to outsource, if you ask me.