I have to subsidize them by maintaining the box so they can save labor and don't have to walk up to my door to deliver spam.

The spam is a profit center for the USPS, it's what allows them to deliver other mail cheaper than UPS/FedEx.

This must vary by location. My box was purchased initially by the neighborhood but is maintained by the USPS. My mom's house gets walk-up service with mail through a slot. Aside from buying stamps or paying to send a package, I have never directly subsidized USPS infrastructure. Excluding the punitive pension requirements Congress uses to make USPS look like a money pit, they'd probably be profitable.

In more rural areas there are not neighborhood boxes like that. Each person is responsible for an individual box by the road.