39 CFR §447.42

1. (a)In addition to the restrictions on political activities mentioned in § 447.51, an employee may not:
   1. (1)Display a political picture or sticker on property owned or leased by the Postal Service. He is not forbidden by this paragraph, however, from displaying a picture, including a personally autographed picture of a political figure in his office or place of work if it has no language in the nature of political campaigning;
   2. (2)Wear a political badge or button while in uniform or while on duty when that duty requires him to deal with the public or be in the view of the public;
   3. (3)Display a political picture or sticker on his private vehicle while that vehicle is being used for official postal purposes.