Hello Mr. Vintage Heirloom,
Takeshi has some great points about priorities, and avoiding keyword stuffing your URLs. I might add, however, that putting category directories in product URLs has two major disadvantages that, in my opinion, usually outweigh the advantage of having those keywords in the URL.
#1 - If the product exists in multiple categories you risk having more than one URL for the product. This can be mitigated with redirects or rel canonical tags, but is still a pain. Here's an example:
http://www.vintageheirloom.com/vintage-chanel-bags/red-2.55-classic-double-flap-bag-1362483150
http://www.vintageheirloom.com/vintage-chanel-bags/2.55-bags/red-2.55-classic-double-flap-bag-1362483150
The bag's URL can be accessed from at least two different URLs (some products could have many more versions) and the canonical tag says that the shorter of the two URLs above is the canonical version. If that is the case the keywords in your /2.55-bags/ directory are useless as a ranking factor for that particular product page since that directory is not in the rel canonical tag.
Yes you can avoid the issues presented by multiple URL versions of the page, and some sites only ever put a product into a single category. However, that does nothing to account for this...
#2 - The deeper your category structure goes the further away from the root your product pages are. I have seen product pages five or six folders deep across entire eCommerce sites because of this. While I don't think the entire site architecture should be completely flat (some sort of taxonomy in the URLs is logical and useful) you don't want your most important pages to be several folders deep either.
I always recommend going with this:
site.com/products/product-name/
Or in your case:
site.com/products/product-name-uniqueID/
Putting the products into the /products/ directory is that level of useful taxonomy I mentioned above. This allows you, for instance, to do a search on Google like (site:domain.com inurl:products) to see how many of your product pages are indexed. The same type of logic is useful when segmenting analytics reports or WMT exports in Excel, among other uses.
Then you don't have to worry about keyword stuffing due to keywords already contained in the category directory portion of the URL.
This is just one person's opinion though. Some may disagree. I just don't find keywords in the URL to be all that important these days compared to other things. It has been spammed to death and thus the importance attributed to that factor has been steadily declining over the years, at least to my observation.
Regarding 301 redirects, they don't really cost you any appreciable amount of pagerank. It truly is negligible as long as you're not going through several redirect hops at once. The key is to make up your mind about your URLs with an eye to the future scalability and useability of the site - and stick with it. One round of redirects will temporarily set you back in the SERPs, but you should bounce back within a couple of weeks (good time of year to do them!) if done correctly.
Good luck!