All, repeat, all federal LEO are sworn to enforce all federal laws, whether the law falls under their department or not. Postal worker are NOT armed. Only postal inspectors are allowed to be armed. Postal inspectors make felony arrest and escort valuable mail shipments. I know for a fact that all U.S. Marshals and all deputy U.S. Marshals are required to be armed at all times because they considered to be on duty at all times. The only exception being on leave in a foreign country, by the way even I had to get permission to leave the U.S. on leave and I was just a pilot.