I'll even spot you one but it does have an exception and if I'm not mistaken purchasing stamps is a legitimate purpose for going into a post office.
39 CFR 232.1:
(l) Weapons and explosives. 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.
Sent from my rotary phone
"The way I see it as soon as a baby is born, he should be issued a banjo!"- Linus Van Pelt
UNIX - Not just for Vestal Virgins any more