I have told people that have asked me, I do not recommend the Post Office. They do not care about the workers and will work you to the bone, because they know you will do the work. Working crazy hours during the whole year, and then even more crazy hours during the holidays. Usually a minimum of 12 during the holidays.
And if you want to go to school, they make it nearly impossible. They don't work with you.
At my office, we get the schedule a day, maybe 2 days in advance if we are lucky. And they only except a request for time off a week ahead of the date you want off. If it is further out, you may as well cancel your plans. They will either forget or say the needs of the Post Office are above your requested time off. And then you just waste your money on your flight, reserved hotel, or whatever else.
Pay is good, but not worth it.
Another thing, I do not see the Post Office being around, at least to the extent it is now, in 30 years when I would be able to retire.
Do not apply at the Post Office, unless it is a stepping stone.