There's nothing new here. It's been illegal to carry firearms on USPS property (not just the building) for many years. The decision simply affirmed the constitutionality of this law. I doubt this case will get to the Supreme Court. Justice Scalia, who wrote the Heller decision, stated in the decision that the Second Amendment did not prohibit the regulation of firearms possession in "sensitive places", such as school and government property. You may not agree with that interpretation, but it's coming from one of the most conservative, pro-gun Justices on the Court.