Yep. I think their obligation is in the hundreds of millions on each Jan 1, before a single stamp is sold or a letter sorted.
My local carrier (rural) was a gem; she lost a package once and we all helped her find it on the side of the road.
When she had cancer, we found out they couldn't replace her, the others had to take her route after theirs.
When she died, we went through about 10 substitutes who were already employees but "brought in" to handle her route and then some.

It sounds like a real personnel mess/business model mess, but I will not blame the union that much.
I have a close friend who just wanted to get out of the Marines and work at the post office. I see his life as my information on this.
After 34 years, he is still on the night shift, still trying to be flexible, and he is not making serious money. Decent, but not over any local average.
He stopped drinking because his job bothers him and he doesn't want to "go postal." We laugh about that, but behind his eyes, he's sad.
I don't begrudge him any of his benefits, because he's definitely not a complainer and I don't see him gaming that system.