I would agree if it's a totally new product. But the links are still going to say
example: shop/viewitem.php?productid=45

For minor changes/additions to your site, there is no need to resubmit a sitemap every time. The search engines will crawl and index your site with or without a sitemap, - or whether or not your sitemap is absolutely complete and up to date.

If you want to draw Google's attention to a specific change or addition you have made, you can always submit the particular page (URL) to them for indexing on their Webmaster web page:
Google Webmaster page (for your registered site) --> Health --> Fetch as Google (here you pick your options...)

It's been my experience that pages that I have submitted for indexing in that manner have been indexed by Google within hours of submittal.

Cool - thanks for the helpful replies...