Always buy rustproofing, if you bought a car crappy enough that the paint job doesn't include anti-rust elements, especially if you live in a location with lots of water-based weather or near the coast.

Why are you adding money to the lowest price they can sell at? That is your target price -- the lowest price they can sell will always be the lowest price that includes a profit (or the smallest loss, for brands like Chevy). Besides, most of the profit comes from the optional packages, which are marked up 200-500%.