Postal unions will not follow rules and after the election is stolen will issue a formal apology.
That apology must be followed by extermination of those involved. Violence for destroying the country is necessary.
The USPS is a Federal Agency.
The Postmaster General is appointed by POTUS (recently, BTW) and provided Advise&Consent approval by the Senate.
The USPS, being of the executive branch, has a Marshall force of inspectors and inspectors.
They are Federal Agents.
They can be directed by POTUS to be completely in charge of all chain-of-custody aspects of mail-in ballots.
If help is needed, Federal Reserve members can be called in by DoD/POTUS to facilitate the effort.