Sen. William Proxmire has awarded his latest "Golden Fleece" award to the U.S. Postal Service, saying it spent more than $3.4 million on a Madison Avenue advertising campaign aimed at persuading Americans to write more letters.

"Spending public money trying to make Americans more prolific letter writers is a dubious project at best," the Wisconsin Democrat said in a statement released yesterday.

"People write letters for many reasons, but I doubt that a public rlations pitch from the federal government is one of them. In my view, this kind of type just won't make Americans type," Promire said.

Proxmire announces 'monthly "Golden Fleece" awards for what he feels is "the biggest, most ridiculous or most ironic example of wasting federal tax dollars."

In addition to the $3,475,600 paid to New York advertising agencies Young and Rubicam and Needham, Harper and Steers, the Postal Service also has spent $774,500 to run ads in three test markets and evaluate the results.

Postal officials are considering whether to spend another $1 million for additional tests in 1978.

Postal Service spokesmen said the purpose of teh tests is to see if advertising can increase the volume of mail. Low volume is one of the causes of the agency's chronic deficit.

Proxmire said, "There is a very great question of whether a government-subsidized agency should be in the game of trying to drum up more business for itself."