An update to this... So I went to talk to the apartment manager this morning, and it turned out that just about EVERYONE got the late rent notices. It turns out that the manager collects everyone's checks and mails them out on the third, and they always get to the home office (on the other side of the state) before the 5th. It was because the 4th fell on a Tuesday and messed up mail that everyone's checks were late, and no one has to pay any fees.

I didn't even think about getting receipts for my payment, although now I'll definitely be sure to request them in the future. Thanks for the tips. n_n