Seconding this recommendation. I'm a Mac developer and sell shareware, but have very little web development experience. I switched from a (pretty thrown together) implementation using Paypal to Fastspring a few years ago, and the process could not have been smoother. Fastspring was super helpful getting me set up, and I had to do very little work to get my existing (PHP) license generator integrated with their system. I've been a satisfied customer ever since.