During most of the post office discussions there is always a dearth of cases. While this isn't exactly post office carry case, it is virtually the same.
Dorosan is a postal worker in Louisiana. One day he left his bag after he had gone home. A coworker found the bag and discovered a twelve round magazine inside. He alerted postal authorities, who on Dorosan's return searched his car with the consent of Dorosan. The found an XD 40 bitone (nice choice!) in the glove compartment. Dorosan was arrested for possessing a firearm on postal property.
Last week, after the Heller opinion, Dorosan filed a motion to have his case dismissed based on Heller. His motion was denied and he was convicted July 2.
Carry at a post office and you are breaking the law.
Here is the transcript of the motion denial.