The U.S. Postal Service expects to have a $400 million surplus at the end of the fiscal year, its second surplus since World War II, Postmaster General William F. Bolger said yesterday.

That surplus, however, aroused questions yesterday during a forum by Ralph Nader's Public Citizen group about Bolger's insistence last year on a postage increase higher than one suggested by the independent Postal Rate Commission.

Bolger had wanted to increase first-class postage to 20 cents, and the rate commission wanted it at 18 cents.

Bolger replied yesterday that the 20-cent rate was necessary to comply with its legal mandate to break even during the first 12 months the new rate was in use. Bolger said the Postal Service had complied with that mandate.

Bolger also said he doesn't expect the 20-cent rate to increase before the beginning of 1984. Last November, postage increased for the sixth time in 10 years and the second time in 1981. In March last year, the cost of first-class postage increased from 15 cents to 18 cents. In November it rose to 20 cents.

The Postal Service had its last surplus--$470 million--in 1979; last year it had a deficit of $588 million.