Post office thing is a fed law. Illegal to drive on their property and store it in your car while going inside or even drive through and deposit mail in the outside drop box at 3am if you want to get down to the technical stuff. Not allowed on any part of their property on you or in your vehicle, period.
This is why I park in the WalMart parking lot across from the Post Office if I have to go there. Pisses me off, ugh. It's only an extra 100 feet or so, but it's the principle of the thing to me, honestly. Not really pissed off, I guess, it's just an irritation, one more indication of how little our government thinks of us and how our elected officials think we need to be controlled.