I would say it's more likely that the hours and wages are the problem.
A few grumpy delivery workers aside, most people in Estonia are nice in general. You should come visit :)
I don't know where you are but consider that the reasons your postal workers are pissed of may go deeper than simply being government employed. Could it be your state-owned services are being managed in way that makes their workers unhappy because they are run by people who think that government services even need to be profitable in the first place?
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.