At this time, the debate seems to revolve around the fact that Postal property is Federal Property which has an across-the-board ban on carrying of weapons on the property.
Recently, the parking lot, weapon secured in a private vehicle while owner inside to do business, was deemed to be exempt from the ban.
Of course, in my experience, when you are a civilian with a Taser, the DA's try to claim the Taser is a lethal weapon. The response the judge upheld for me was that the FBI deemed them non-lethal, therefor not regulated under lethal weapon laws.
I'll dig up the discussion thread if you cannot find it for yourself