I work for the Postal Service and have been shorted pay what can I do?

In the past year I have been shorted my pay for whole days of work. at this point they owe me 9 days of work more than half the days are over time so it ads up to a considerable amount of money. I have tried to be reasonable about this situation. I stayed late on my own time and researched the payroll errors made copies for my boss and still have not been compensated. I am fed up with local management not acting and getting me paid. Additional info work forty plus hours a week and have done so for 4 years not and still do not qualify for employee benefits such as health insurance and retirement any lawyers looking for that big fish to make there name the Postal Service seems like a 8 ton gorilla in the room.I work for the Postal Service and have been shorted pay what can I do?
if your immediate supervisor wont fix it, go to their supervisor. I would tell your supervisor that you are going to have to take action first, maybe that will make them do something about it. Talk to Human Resources. you didn't mention what position you are in. you should know what you qualify for and what you don't, after any amount of time. Human Resource department can help you with that.