Carry a gun, even with a state issued permit, onto any PO parking lot in the United States is a felony.
https://www.reuters.com/article/usa-postoffice-guns-decision/no-guns-on-post-office-property-says-u-s-appeals-court-idUSL1N0ZC2QF20150626
So, again, the post encouraging you to ignore GFZ...