First-class postage will rise to at least 28 cents and perhaps as high as 32 cents in early 1991, Postmaster General Anthony M. Frank said on NBC-TV's "Today" show. He promised that the increase would be accompanied by improvements in the U.S. Postal Service, saying that about 40% of letters already are handled by automation and the benefits of automation will be felt in 1990. Frank denied that increased automation will reduce the number of postal jobs because the workload is increasing. Rate increases must be approved by the independent Postal Rate Commission. The last hike was in April, 1988, when the rate went to 25 cents from 22.