Milton Avery is one of the most well-known of the post office muralists. Like many artists, he worked through various genres throughout his career, but he is best known for his flat (intentionally lacking visual depth), abstract works. You can see examples of his work at the websites for the Smithsonian American Art Museum, the Whitney, the Met, and the National Gallery.
Avery created one post office mural: Landscape (Rockville, Indiana).