Do yourself a favor and download WooCommerce instead.
Not only is this plugin buggy, but there's nowhere to report a bug. Their forums are all "user supported," so don't count on any help from there. You can, of course, pay $47 for each question (or bug report) that you would like to bring to their attention. I had to raise a PayPal dispute once for this "service," as my problem I asked for help with was ignored - until I filed the dispute.
The documentation is awful. Their "WPEC Consultants" won't bother to return e-mails or acknowledge them, so even if you're willing to pay someone to fix your store, you won't be able to find anyone to do so.
The store I run on this plugin is constantly losing sales because of a buggy shopping cart. My WooCommerce store has never had a problem - and I will be switching the other over to WC asap as this is how I earn my living, and I refuse to keep losing money each month.
Save yourself the aggravation and pass this one up.