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.
[I'd actually recommend talking to FastSpring - they used to provide a concierge service to setup the order form for you, and they have a system where they can ping a licence key generator on your server (eg in PHP), much like what Kagi did. I'm happy to try to answer questions if you or anyone else gets stuck, contact details in my profile.]