USPS delay- happened to anyone else?

I handed a first class package to my mailman on Tuesday. Tonight it is finally showing up in tracking. It shows as being accepted at my local facility at 5 PM on Sunday, which is impossible since the post office is closed on Sunday. A similar thing happened with my previous first class package. Even if I do happen to get enough transactions to retain my top rated status, I'll lose it on shipping time. And I risk getting negative feedback as well. I can't afford to switch to priority as these are cheap items I'm selling for under $15, including postage.

I don't know what to do. If I complain to my post office, it could backfire. Is my mailman keeping my packages in his truck for days before turning them in? Or is a policy of my local post office, the same one that refuses to scan packages when they are dropped off?