I've just got the most excellent Ubercart ecommerce system to work with Paypal for paying and then auto-downloading digital media (eBook).
I thought I would jot down the process, as a memory jogger for me and if it helps anybody out there in inter-web-cyber-highway-net-land then it's worth the wasted minutes of typing ;)
So lets start with the basics, obviously replace http://the-pc-workshop.com with your own website name... duh!
Ubercart is a shopping cart and e-commerce suite built on top of the leading open source CMS, 