Its frustrating, that unless you open your store with the proper settings on and a dedicated domain purchased and pointing at the store and a dedicated ssl cert bought and installed then a ProStores store will have duplicate content issues because of the shared urls. This doesn't have to happen.
I've been working with a client who just watched her monthly sales fall 85% because google finally caught her store03 addresses and has lost her good positioning for products that were bringing in good money. She got ahold of us and we have addressed all the issues we can and are now waiting for google to pick up on our changes and are hoping for the best.
This is just one example!
Because ProStores doesn't allow merchants to have access to htaccess, believe me I understand why as its too dangerous for the untrained, merchants can't address these as best practices would demand.
Now, there are a handful of other options, but they just aren't as good and as permanent as a proper 301 redirect or better yet no shared urls at all.
If I had my way I'd ask ProStores to require a domain name while registering, let them buy it through ProStores or enter it in if you already have one, mediatemple does this and its fantastic, then require a dedicated ssl cert, give them a discount of some sort since you can get bulk prices I'm sure.
Most more modern e-commerce systems have this done and thereby don't have the crippling duplicate content issues that 90% (my estimate) of ProStores merchants do.