I use paypal. It's quick to setup and has Paypal IPN which is essentially a call back to your system to let it know that the funds have been received or an order has been made. You can also set up free trial periods and things like that. It's quite easy to use.

Thanks for the link. Have you worked with this company?

I do not work for this company, though I use their services.

Erlang may be the death of this question.

Thanks for your response pj. Paypal seems to get a lot of negative press- is it unjustified?

Erlang is also the death of a lot of fluff articles of late.

The thing with Paypal is that they aren't quite so flexible and they have high fees, 3 to 4% or more. My only complaint was that they are very pro-consumer, meaning they side with the credit card company when the purchaser of your goods complains, but this is normal. There are charge backs with credit cards too if you do your own processing and payment management.

Paypal is the only payment system I've used. I wish their system was easier to customize.

You may want to check out Amazon FPS, but I haven't used them.