(emphasis added)
Title 39 - Part I - Chapter 4
§ 410. Application of other laws
(a) Except as provided by subsection ( of this section, and except as otherwise provided in this title or insofar as such laws remain in force as rules or regulations of the Postal Service, no Federal law dealing with public or Federal contracts, property, works, officers, employees, budgets, or funds, including the provisions of chapters 5 and 7 of title 5, shall apply to the exercise of the powers of the Postal Service.
And the USPS regulation……………….
1.4.6. Weapons and Explosives
No person while on Postal property, or while performing services under a Postal contract, shall carry firearms, other dangerous or deadly weapons, or explosives, either openly or concealed.
As in.......the USPS as what is called a "quasi-governmental independent agency" (like the CIA or NASA) can pretty much be a lone wolf, and do their own thing as they see fit as voted on by their Board of Postal Governors.
We've always said no-no to carrying in the Post Office.