Actually, if you look at the fine print on any application for a carry permit it will have a specific list of areas where the permit is not valid. I recall some of the prohibited locations for Florida permits are schools, government offices, and any establishment that serves alcohol. (I may be redundant here since the latter location probably applies to the two former locations
) As for the Post Office, they put that sign there so people are on notice that Federal jurisdiction applies and it wouldnt be a local charge if you were caught carrying on the premises.