Thanks for your notes here on getting this set up -- it's been very useful as I've been also setting up a CD shop. In regards to sorting the output of relativity, I discovered you don't need a bounty, the ability is already there! Under node relativity display settings (admin/settings/relativity/display) there is the rendering option for children nodes. And since you can use a view as the rendering option you have all the sort power of views at your fingertips.

An example view that I created for my track previews has the following: Page View enabled in Table mode, Fields of Title and Audio player, Filters is Published and Node type, Arguments is "Relativity: Parent Node ID" (this is the really important one!), Sort is "Product: List order". Works like a charm.