[Cletus.D.Yokel]

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.

[jacknhoo]

“They can be directed by POTUS to be completely in charge of all chain-of-custody aspects of mail-in ballots.”

Just how would one go about doing that? You going to have them visit every household multiple times to collect ballots and verify the persons mailing them?

[Mr. Jeeves]

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.

Just imagine the delicious screams from the Left if Trump proposes this... :)