NavyLCDR
member
NOTE 2: My general detached view of all this is that Post Office officials generally don't want you to carry in their facilities, but there really isn't much they can do about it except huff and puff and make it sound like you can't. <This is not legal advice, only my own rather acid comment on it all.
I believe the fine is $500 and up to 30 days in jail:
Title 39: Postal Service
§ 232.1 Conduct on postal property.
(p) Penalties and other law. (1) Alleged violations of these rules and regulations are heard, and the penalties prescribed herein are imposed, either in a Federal district court or by a Federal magistrate in accordance with applicable court rules. Questions regarding such rules should be directed to the regional counsel for the region involved.
(2) Whoever shall be found guilty of violating the rules and regulations in this section while on property under the charge and control of the Postal Service is subject to a fine as provided in 18 U.S.C. 3571 or imprisonment of not more than 30 days, or both. Nothing contained in these rules and regulations shall be construed to abrogate any other Federal laws or regulations or any State and local laws and regulations applicable to any area in which the property is situated.
(q) Enforcement. (1) Members of the U.S. Postal Service security force shall exercise the powers provided by 18 U.S.C. 3061(c)(2) and shall be responsible for enforcing the regulations in this section in a manner that will protect Postal Service property and persons thereon.
(2) Local postmasters and installation heads may, pursuant to 40 U.S.C. 1315(d)(3) and with the approval of the chief postal inspector or his designee, enter into agreements with State and local enforcement agencies to insure that these rules and regulations are enforced in a manner that will protect Postal Service property.
(3) Postal Inspectors, Office of Inspector General Criminal Investigators, and other persons designated by the Chief Postal Inspector may likewise enforce regulations in this section.
If you commit a crime while armed on postal property, then you get the sentence enhancement called for in 18 USC 930 (b) and/or (c).
Good luck.