According to a posting in the Google Webmaster blog, their latest recommendation is NOT to rewrite dynamic URLS into static ones.
Rewriting URLS is supposed to make it easier for your users to read and remember URLS but I don't really think it's that much an issue in a dynamic shopping page. They'll just copy/paste or bookmark the URL anyway, so what's the problem? Seems like a lot of work for not a lot of gain, but that's just my opinion.
Official Google Webmaster Central Blog: Dynamic URLs vs. static URLs
And a related article at Search Engine Land
Google Says, Don’t Rewrite Dynamic URLs To Static URLs
Above all, just make sure they can find the page and let them spider it. If you've done your job, it'll rank well.