Pros
If you are union after being past your 90 working day probationary period, you have to really mess up multiple times (5 write ups or one backing accident) to loose your job. [ semi job security] If you aren't getting enough hours at your office (in a world where) you can work in other offices that need your help. There isn't many pros to be completely honest. If you don't have a family, or school, children.. Anything you might miss out on then you will do just fine.
Kontras
Package overflow. You will be sorting, casing, pulling down, loading un loading and delivering mail with the "more important" package load of what is comparable to the standard UPS day. You essentially must work every Sunday for Amazon package delivery. Management like to play "games." A manager didn't like me so She didn't pay me a works worth of pay. I am still waiting 2 months later after going through the union and having to do all of the communication myself, to get that pay. Your daily rate is evaluated, this means that if you are working "route 22" for example and it evaluated 7 and a half hours and you work 10 hours completing the task... You are paid 7 and a half hours. The evaluation is not inclusive to mail volume, or package volume.. Un-authorized dismounts such as entire streets blocked by parked cars are irrelevant to the evaluation. Management , oh man. You are 80% of the time, a worker.. Not a human being staffed to work there. Very few managers are what seems to be of the human kind while the rest seem to have little to no management experience and ended up in management after hurting themselves delivering mail themselves. No life work balance until you stand up for yourself and say no. No benefits until you are a "regular." No real opportunity for growth.