Older is often better. Most of the newer post offices that I know are rather nondiscript utilitarian buildings. I think quite a few of the more attractive buildings flowed from New Deal efforts to stimulate the economy and provide employment for architects and artists. Some of the much older buildings in larger cities are also pretty nice, witness a couple of examples like NYC and LA in your link.

The downtown Pittsburgh post office is a rather nice Romanesque structure on one of the minor avenues:



Big Al