This is from a thread here on TGO that Fallguy posted 8/27/2008. This is where I got the information about having a weapon on PO property (parking lot).
http://www.tngunowners.com/forums/handgun-carry-self-defense/11352-updated-info-post-office-carry.html
39 CFR 232.1 - Conduct on postal property. - Code of Federal Regulations - Title 39: Postal Service - vLex
Title 39 - Postal Service
Chapter I - United States Postal Service
Subchapter D - Organization and Administration
Part 232 - Conduct on Postal Property
232.1 - Conduct on Postal Property.
Paragraph L
(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.