It's entirely possible that the Postmaster is just giving you guys the run around because he doesn't want to deal with it.
When your wife submitted the package, did she submit a doctor's note affirming your mother's disability? I thought you must have it in writing from the doctor. If you guys didn't have that, I'm guessing the post office trashed your request and is just telling you, "Yeah sure, everything's fine"....because that's the USPS way. The internet is brimming with news stories of this exact thing happening.
I would say to call the hotline for the OIG, who should still be able to refer you to the Office of Consumer Advocacy if they feel the case lacks merit. I say call the OIG just because the chance is slim, but there is in fact a chance they might have something going on where they might want to hear from you. Remember that federal OIGs don't just conduct criminal investigations, but employee misconduct investigations as well. OIGs are split into several different divisions, and only one is criminal investigations. Just manage your expectations, as The French mentioned...
In the meantime, move into the 21st century and abandon the archaic practice of receiving and paying bills via mail.