Hi LW,
What system is backing the online store? Are you using a CMS-driven e-commerce solution?
My suggestion would be to create an automated sitemap for the products. Pay careful attention to the priorities you assign and the update frequencies. (Hourly/daily is fine) I definitely think that you'd be spending far too much time on updating a sitemap if you had to do it manually.
This method will result in you having a more accurate sitemap on crawling.
Also, if you are planning on offering the same project in future, it might be an idea not to remove the product altogether, but rather have a page saying "This offer is currently not available" or something along those lines.
Another option might be to have an archive category of products, where all your expired offers can be placed, not available for order. This could allow you to keep your indexed pages, avoid 404s as well as use the product pages to direct new visitors to related/newer products should they see the products in the archive.
Just thinking out loud. 
I'd be interested to see the website and the solution that you do eventually implement.
Regards
Mark
