Don't use this plugin. It will break. You will lose sales.
As a developer, WPEC is an absolute nightmare. Getting it to work takes a lot of effort, and every update changes the core of the website substantially. Good lucky trying to customize any of those template files!
I've used this on three different websites, all of which are currently outdated because it's too much of a hassle to update.
What broke this time? You can't add products to the cart. Not even with other plugins disabled and using a stock theme. It's so terrible. And look at all those other 1-star ratings.