The U.S. Postal Service lost $306 million in fiscal 1980, adding new pressures for the corporation's request for a postal rate increase.

The loss is about half of what postal officials had expected. In fiscal 1979 postal service reported a $470 million profit -- the first in its history as a government-backed corporation.

Postmaster General William F. Bolger, outlining his organization's annual report during a press conference yesterday, said if the postal service gets the rate increase it has requested -- including a 5-cent increase in first class stamps -- and "if we can arrive at a reasonable contract agreement with our employe unions this summer, we have every reason to believe we can continue to hold rate increases to at least three-year intervals." The previous rate increase was three years ago.

The independent Postal Rate Commission is expected to announce its recommendation on the Postal Service's request on Feb. 20, and the Postal Board of Governors will then make the final decision. Bolger said he expected the new rates to begin in March.

In addition, Bolger said he doesn't know how the postal service will be affected by Reagan administration budget cutting or moves on Capitol Hill to eliminate the proposed expanded nine-digit zip code.

"I don't expect anyone to look at the new numbers as the greatest thing that's come down the pike," Bolger said in response to a question about the much-criticized new zip code system. "We want to get around to many of the Uncle Joes of the world" and explain that the expanded zip code isn't hard to use and isn't mandatory, Bolger said.

"We have to get automation whether we get a nine-digit zip code or not," Bolger said. The Postal Service plans to phase in the new zip starting in June.

In addition, Bolger said the Postal Service won't have to eliminate one day of service to make ends meet.

Mail volume increased last year by 6.5 billion pieces to 106.3 billion letters and parcels. For the first time in 14 years parcel-post volume increased 2.4 percent. "It was only a small gain, but it is symbolic of the progress we are making in our efforts to be competitive with Ups [United Parcel Service] and other package delivery services," Bolger said.

Volume of Express Mail, a relatively new service that provides same-day or overnight service at an extra cost, also increased by 43 percent, Bolger said.

First-class mail volume rose 4.1 percent to 60.3 billion pieces, third-class mail volume increased 10.4 percent to 30.4 billion pieces and delivery of periodicals rose 10 1/2 percent to 975 million pieces, Bolger said.