So you've completely missed the entire discussion about the fact that Title 18 930 doesn't pertain to post offices and that there are no federal employees at a post office so there is no federal facility to consider.
Also missed the fact that the CFR has already been posted, by me, about carrying and the "except for official purposes" part where if I'm going in to buy stamps, I do believe that's an official purpose for a post office.
Plus there is at least one court case where a conviction for firearm in a post office parking lot was thrown out and zero for a conviction of that sole charge. NOT and "and a" charge, a sole charge.
There is a single case where a postal employee had a firearm in the "employees only" parking lot and was convicted of that but that was a limited access lot and not the general public lot so that's a different scenario from me going in to get stamps.
You might want to actually read the thread to see if what you're posting was already discussed.