These are sometimes referred to as WPA projects (The Jasper County Interim Report makes this mistake), but they were funded by a different program, one in the Treasury Department:
Often mistaken for WPA art, post office murals were actually executed by artists working for the Section of Fine Arts. Commonly known as "the Section," it was established in 1934 and administered by the Procurement Division of the Treasury Department.Here are some more information and perhaps some better pictures of the mural. It seems that it has had some restoration work done; the company that did it has done a great many of these New Deal Post Office restorations.
I looked for more information about Costigan and did not find a lot. He seems to be a pretty minor artist, and his art, mostly prints, sells at auctions for hundreds of dollars, not thousands.
Next time you are at the post office, take a little time and appreciate the interior. It seems little changed from what it was like when it was first built. It is very 1940s--a living museum in a way, in an age that no longer relies on it to deliver messages.
If you visit old post offices in other cities, be on the lookout for similar art work.
Update: According to the numbers on the side, this is post 100 for Rensselaer Adventures.