Postmaster General William F. Bolger, citing inflation as the cause, yesterday recommended raising the price of a first class stamp from 15 cents to 20 cents starting early next year.

He also proposed raising the price for post cards from 10 cents to 13 cents and increasing commercial mail rates.

But rates for nonprofit and some newspaper mailers would decrease under Bolger's proposals, and persons mailing Christmas cards could avoid the full increase in stamp prices.

Bolger said he will recommend a 2-cent Christmas discount from the new rates for mail deposited between Dec. 1 and 10 that wouldn't necessarily have to be delivered until Dec. 25.

This Christmas rate wouldn't apply until 1981, Bolger said.

The entire rate proposal is subject to approval by the independent Postal Rate Commission and wouldn't become effective until next February or March, Bolger said.

The first class recommendation would bring the largest numerical increase ever for postage rates and would give the Postal Service small budget surpluses in 1981 and 1982, Bolger said.

The Postal Service, which had its first surplus last year since 1946, will have a budget deficit of $2.3 billion in fiscal 1981 if the rate increases are not approved, a Postal Service spokesman said.

The recommendation was made on the basis of the Postal Service continuing its six-day mail delivery and receiving its public service appropriations from Congress relatively uncut, the spokesman said.

Congress is considering cutting those appropriations to help balance the federal budget. Bolger has warned that if appropriations are cut too much, he will recommend the elimination of one day of mail delivery.

The proposed increase is the first since the price of first class stamps rose from 13 cents to 15 cents in 1978. If the rate of inflation hadn't "shot up" since the last rate boost "we would have been able to keep the current rates in effect until 1982," Bolger said.

If the Postal Service weren't counting on significant increases in efficiency, Bolger said, he would have filed for an increase to 22 cents for a first class mailing "merely to keep up with the rate of inflation."

"If there was any way we could avoid any increase we would take it," Bolger said, "because the plain fact is that higher rates do not in any real sense, benefit the Postal Service. In fact, to the extent higher rates discourage people and businesses from using the mails, they invite inroads from our growing and increasingly aggressive competitors and this weaken the Postal Service."

"In short, if there is a villain in this story, that villian is inflation," Bolger said.

The Postal Service's recommendations go to the Postal Rate Commission, which will hold hearings on the proposals and make a decision on the request within 10 months. The Postal Service Board of governors may then approve, reject or modify that decision.

Bolger said he is confident that the rate commission will accept the proposal. He has previously said that a rate boost was planned for 1981.

Bolger said yesterday that almost all of the rate request can be laid to the Postal Service's two largest expenses, which consume about 93 percent of its outlays: the wages, salaries and fringe benefits for 665,000 employes, and the operation of 240,000 vehicles.

For example, Bolger said that February's rise in the consumer price index alone added $11 million to the Postal Service's payroll costs, because of cost-of-living adjustment clauses in its collective bargaining agreements.

When the price of gasoline rises 10 cents, Bolger continued, $30 million is added to the Postal Service's annual fuel bill.

From 1978, when the 15-cent rate was instituted, through early 1982, the end of the first year of the proposed new rate, inflation is expected to be 48 percent while postal rates would go up 28 percent, Bolger said.

Bolger said he was confident another rate increase wouldn't be necessary for an additional 2 1/2 years. When asked how confident he was, Bolger replied: "As confident as you can be of controlling inflation."

Other aspects of the rate request would increase by 1.9 percent regular second class rates, which apply mainly to mass circulation newspapers and magazines, decrease the third class rate for nonprofit bulk mailers by 5.7 percent, reduce by 6.2 percent the second class rate for nonprofit publications and increase by 17.7 percent rates on regular third class bulk mail covering circulars and catalogues from profit-making organizations.

Commercial users who separate their first class mail by zip code and deliver it to the post office could pay as little as 16 cents.

Costs of parcel post material would increase an average of 8.4 percent.