The USPS in Dallas during the times of the JFK Assassination had a few FBI informants present, this we can deduct from the documents in the Harry Dean Holmes segment. Informants with code names T-2 (Harry Dean Holmes), T-6 (Roy R. Armstrong) and T-10 (Harry Dean Holmes again). Why Harry Dean Holmes has been ascribed two code names is a mystery.
The USPS got its role in the JFK Assassination mostly due to the post office box Alek Hidell had.