Yesterday, First Lady Nancy Reagan presented a historic preservation award to the mayor of Frederick, Md., for helping save the town's 65-year-old post office.

Today, the U.S. Postal Service will announce that it is advertising for bids to demolish the building.

The award, one of 25 given by the National Trust for Historic Preservation, commended Frederick Mayor Ronald Young for his successful efforts in guiding the restoration of one of Maryland's historic towns and for "saving the 1917 post office at the 11th hour, a welcome turnabout for a city that has lost some important landmarks."

Sheila Tate, Mrs. Reagan's press secretary, said the First Lady was unaware of the plan to raze the building, but noted that the Postal Service is a quasi-federal agency that operates more independently than other government agencies.

A spokesman for the postal service said last night that while the timing of the two announcements was "ironic," it plans to go ahead with demolition of the Greek Revival structure.

Mrs. Reagan, appearing at the trust's 36th annual meeting at the National Gallery of Art, praised the nonprofit group "for all the good work you do. We Americans are very thankful that you've stopped the bulldozers and saved some of our great architectural landmarks."

Mrs. Reagan was given a standing ovation by trust members, although there were several references during the day to the "cataclysmic effect" and "financial difficulty we're faced with" because of proposed White House budget cuts. Mrs. Reagan's husband has recommend a zero budget for federal historic preservation programs in his 1983 budget.

The Frederick post office, while not one of the city's landmark buildings, was thought to have been saved last fall when Postmaster General William F. Bolger came to town to dedicate a historic Frederick stamp and promised Mayor Young that the old post office would be "saved"--that the city could lease it or buy it.

However, postal officials apparently persuaded Bolger to renege, although a brief and unsuccessful attempt was made to lease the building. Last month postal officials announced they would proceed with their plan to demolish the building and turn it into 64 "badly needed" parking spaces for employes and customers, including the handicapped.

The old post office became surplus in 1977 when a new and modern postal facility was opened next to it in Frederick's downtown historic district. There are 169 government parking spaces for the new building, with patrons expected to park on surrounding streets. Of the 64 additional spaces, 22 would be reserved for patrons, postal officials said yesterday.

Disputes between Frederick and the Postal Service are nothing new, having gone on almost continuously now for two decades.

In the early 1960s, the Post Office Department (it had yet to become the quasi-independent Postal Service) demolished almost a block of historic buildings in Frederick to make room for a modern facility. But it then announced it preferred an out-of-town site, outraging local officials and Maryland congressmen, who intervened and forced construction on the downtown site.

The Postal Service has been trying to demolish the old building for the last six years. In 1977, under pressure from Congress and federal and state preservation officials, it agreed to attempt to sell the building to a private developer. However, no legitimate buyer offered the appraised price of $121,000, and last year the Postal Service declared it was free to demolish it.

Mayor Young, who contends the Postal Service never made a "good faith" attempt to sell the building, was elated last fall when Bolger agreed to lease or sell the two-story building to the city. He now accuses Bolger of welching on his promise.

The cost of demolishing the old post office and building a parking lot, and a small canopy over some parking spaces, is estimated by postal officials at $160,000 to $250,000, twice the amount it apparently was willing to sell the building for.

Bids are to be opened June 2, with demolition and parking lot to be completed within 105 days.