I'm in Poland, but I'm from Italy and I've lived in the U.S. for a few years. Worst post offices in Italy by far, but Poland and U.S. pretty similar.
Other low-paying jobs don't seem to generate the same amount of unhappiness.
I have no idea what the reason is, to me it's just that governments can't do anything right because they're too big with no oversight.
That's why I was complaining about socialism, anywhere I've been where the government runs more than just post offices it was hell.
In Poland, all these Soviet buildings and if you look at old pictures of people standing for hours in queues for bread. Truly horrific. I was recently in Cuba and even if they can't talk about it many people told me they would flee right away if they could, but the government doesn't give them passports. Socialism destroys everything.
Source on 'poorly paid'? In the US starting salary for a USPS carrier is $24/hr. FexEx starting pay is $17/hr.