39 CFR part 232-1
(l) Weapons and explosives. Notwithstanding the provisions of any other law, rule or regulation, no person while on postal property may carry firearms, other dangerous or deadly weapons, or explosives, either openly or concealed, or store the same on postal property, except for official purposes.
Penalty…
(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.
The fine… (I thought I found at one point that the fine was $50, but can't find that now.)
(b) Fines for Individuals.— Except as provided in subsection (e) of this section, an individual who has been found guilty of an offense may be fined not more than the greatest of—
(1) the amount specified in the law setting forth the offense;
(2) the applicable amount under subsection (d) of this section;
(3) for a felony, not more than $250,000;
(4) for a misdemeanor resulting in death, not more than $250,000;
(5) for a Class A misdemeanor that does not result in death, not more than $100,000;
(6) for a Class B or C misdemeanor that does not result in death, not more than $5,000; or
(7) for an infraction, not more than $5,000.
Another quote for 39 cfr 232-1 quoted it this way…
(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 fine of not more than $50 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 of any State and local laws and regulations
applicable to any area in which the property is situated.