I would double-check Q2770BX's answer - I believe that you WILL need your passport as you can't get into the Presidential Office building without it and I don't think you can get into that post office without going through the security check. I was there in October and, with guards holding machine guns all over the place, had to show my passport (I don't think a regular ID will be accepted) as well as go through metal detectors to get into the building...the security was very strict.