Archibald Motley is one of the most famous of the post office muralists that I’ve encountered to-date. He is also, if memory serves, the first African American post office muralist I’ve come across. Motley was an important figure in the Harlem Renaissance. You can see several of his works on WikiArt and the Whitney’s website.
Motley created one post office mural: Stagecoach and Mail (Wood River, Illinois).