IF you are collecting a retirement check from the Military. You ARE NOT eligible to buy your time back into the USPS retirement system, since you are already being paid for that time.
If you could, and it is the same formula as Civil Service, it would cost you aprox 4% or your gross income that you made during your entire time in the Military. You would also have to give up your current retirement benefits if you bought into the USPS retirement system, It is not worth the effort.
The system is for people that did for example one enlistment and want to get credit for that four years toward retirement. Again it costs about 4% of what you earned while in the Military.
Retired AF SNCO, Former Civil Service. WG-10