The fewer directories in the URL, the more weight the page will carry.
Google looks at how buried a page is when assigning page rank, so you definitely don't want to go more than 3 levels deep.
If you've got a dynamic shopping cart that's database driven, you might want to wait before implementing mod rewrites because Google is smart enough to read querystrings.
You might want to simply add a value to the querystring called "colour", "fabric" and/or "item", then assign each one a value like the model number or product name, anything that would relate to a targeted search query that users are likely to type in when looking for your products.
Script the cart to always build hyperlinks like this and you'll accomplish exactly what you want to do.
HTML Code:
<a href="http://www.somewhere.com/product19.html?colour=black&fabric=leather&item=corset
As long as you've optimized your product pages properly, a search for "black leather corsets" makes this a highly relevant URL.
If you do it like this, it won't matter how many new products you add or remove.
.02