It is illegal to carry in a post office: the post office is a federal building. However, according to Gutmacher's Florida Firearms: Law, Use & Ownership, which covers state and federal laws, the post offices set up in card stores or Mail Box Etc.s are considered "contract post offices" and wholly private. You should be fine to carry there. Maybe that's what your instructor was talking about.
What I find interesting on this and other forums that discuss concealed carry is that there's an increasing trend for folks to casually recommend violating laws while touting the rights of law abiding citizens to carry. Really bad form. I also find it scary that many folks (not necessarily those posting here) have their concealed weapons licenses and/or permits and have not taken the time to learn the laws and rely on internet threads for basic information and advice, some of which, as evidenced by this thread, is just plain bad. It's a right to carry a weapon, but it's also a huge responsibility. C'mon people.