Zen Cart (Score 1)

Its not my favorite software ever and is suffering from a sever case of featuritis, but you can do this with Zen Cart (http://zen-cart.com/). Its an open source shopping cart writen in PHP that allows you to sell downloads. When a user buys a download, they are given a custom URL to download the files and this URL can expire after a certain time period. I set this up for a friends record label, and besides the need to strip out all the extra features its been working fairly well. And you can sell t-shirts from the same store.