I have a pretty good history of the Old Post Offices of Fairfax and actually if you were to say that the Post Office was in any one of the buildings remaining, you would probably be right. If you click on the following link, you will find some photos and data on the Post Offices of Fairfax, perhaps even some you have never heard of:
http://www.vtgrandpa.com/forum/index.php?topic=112.0 (http://www.vtgrandpa.com/forum/index.php?topic=112.0)