The Gramm-Rudman-Hollings budget-balancing law and a shortfall in revenues may force the Postal Service to raise the price of stamps before it had planned, according to the No. 2 postal official.

Jackie A. Strange, deputy postmaster general, said in an interview last week that in this fiscal year the Postal Service faces a $32 million cut under the new law. In addition, the quasi-private agency has lost another $65 million because Congress failed to put into effect a rate increase for nonprofit mailers at the beginning of fiscal 1986 last fall; instead, the rise became effective Jan. 1.

Strange said that, if the budget cuts go into effect, "we will be forced into filing earlier than we had hoped we would file" for a rate increase in all classes of mail. However, such a request is so complicated, she said, that it would be about 18 months from the time it was made to the time the Postal Rate Commission could approve it.

Casey, appointed last month, has said that he intends to act only as an interim postmaster general until a permanent successor can be found. Thus, many of the changes he envisions probably will be carried out by Strange, who began her career in 1946 and worked up through the ranks.

Strange, asserting that the Postal Service's efforts to improve its finances will be stymied by the budget-balancing law unless postage rates are raised, said: "We want to support the president on cutting the deficit , but we also want to be realistic."

The agency lost $251 million last fiscal year. Although the Postal Service has a $466 million surplus so far this fiscal year, that figure reflects the Christmas season, the most profitable part of the year. The slow summer months will deplete the surplus, Strange said.

Strange also outlined these efforts to improve the Postal Service:

*Express Mail delivery will be guaranteed for the next morning instead of afternoon, and the number of boxes will increased.

*Ten groups will be created to study topics ranging from management structure to new technology.

*The Postal Service is selling its $1.7 million Cessna jet on the advice of Casey, who said commercial airlines are good enough.