Stanley E. Cohen is mistaken when he says the delivery of first-class mail is merely a sideline activity for the U.S. Postal Service {letters, July 6}. He is also incorrect when he charges that most of the operating costs for advertising mail are charged to letter writers.

Contrary to Mr. Cohen's opinion, the U.S. Mail is not primarily "a conveyor of advertising material and routine commercial paper." Last year, the Postal Service delivered 147 billion pieces of mail, of which 51.7 percent was first-class mail. Third-class mail, which includes advertising mail, accounted for 37.4 percent. First-class mail generated 65 percent of our revenue, compared with 19 percent for third-class mail.

Under the postal reorganization act of 1970, each and every class of mail is required to pay its own way. Advertising mail qualifies for a lower rate because, by regulation, the mailer must perform most of the sorting and preparation otherwise done by the post office. The savings to the Post Office are passed on to the mailer. In addition, the user must mail in large quantities and pay certain fees in order to use the service. By the way, in the Postal Service we call this type of mail "Bulk Business Mail," which is a more accurate description.

Although Mr. Cohen is mistaken in his assertions of preferential handling for advertising mail, the thousands of users of third-class mail are valued Postal Service customers. They spent almost $6 billion with us last year. Third-class is the fastest-growing class of mail, which indicates that it is indeed useful and worthwhile to mailers and the public. Direct mailing supports a large industry.

Advertising mail helps support the postal system just as newspaper advertising makes a paper like The Post available for 25 cents, which is far less than the cost of printing and distribution, and radio and television advertising enables broadcasters to provide free programming to the public.

One final note: Mr. Cohen refers to the "once-proud U.S. Mail." Let me assure him that, as the world's largest postal system with the lowest rates of any industrialized nation, and as the only organization in the United States that delivers to every address in the country, the U.S. Mail -- the Postal Service -- is still proud. ARNOLD L. DAITCH D.C. Postmaster Washington