"Also, they charged me for two even though I only bought one. I didn't notice because I was buying groceries as well."
Nothing you can do about that now.
As for returning used makeup, I've never tried to with Wal-Mart, but other grocery stores have allowed me to do so in the past (on multiple occasions). I've returned used foundation a few times (especially back when I was looking for the right brand) because the product has been crap, and they accept my return, even though I'd obviously opened it.
There's really no harm in trying. Unless Wal-Mart differs from other grocery stores, they should accept the return.
Drugstore brands really need to take a page out of Sephora's book and allow grocery stores to place testers out for all their products.