Recently I have launched a new CAD software site for a client using WordPress as the backend. This site is using Foxycart as an e-commerce platform. Foxycart offers a clean one page checkout that integrates quiet nicely using WordPress Custom Fields.
When a product entry is created you simple give it a price, our price (If on sale) and a few other fields and your done! Foxycart runs about $19 and includes SSL (encryption). Ask me about setting up your store using WordPress.