I downloaded this free plugin and within a few hours paid the (tiny) upgrade fee to the "Pro" version. I could not be happier.
The thing I love about this plugin is that it generates shortcodes for each product, with a button directly in the rich text editor. This has allowed me to layout my store how I want it - using a theme of MY choice! No more working around the "complete store in a box" approach that the other plugins take.
The feature set of Simple E-Commerce Shopping Cart is huge, and the set Pro version features is enormous. There is so much power packed into this plugin, including a free iPhone App for notifications and monitoring sales.
Best of all, I needed a custom feature added. So, I contacted Niaz and he delivered quickly and on budget.
I'm serious impressed with this plugin. Get it.