Not sure if there were more, I remember two.

He worked at "Kramer Business Machines" in the out of retirement episode.

On another episode (early one) he said "There was a reason I sold more women's wear than anyone else in this state".

Businessman slash salesman was what I assumed based on what we knew.
I can't think of any other occupations that were listed, other than that he was a minor league baseball player and a flagpole sitter in the 20s!

In the comics he was retired from the post office, I think, but dunno if it's canon.

As far as the show goes, no. No mention was ever made of either Wilson brother working for the post office.