President Carter's proposal for a 13-cent "citizens stamp" has been turned down by the Postal Rate Commission in a tentative dicision that could mean introduction of a 15-cent-per-ounce first-class mail rate as early as late May.

Informed sources said the independent commission rejected the special rate in a 3-to-1 vote, with Chairman Clyde DuPont joining the majority.

Former Postmaster General Benjamin Bailar endorsed the White House citizen rate concept last year as part of a $1.65 billion postal rate increase proposed by the Postal Service.

Under the plan, personal letters mailed by individuals would continue at the current rate of 13 cents for the first ounce, possibly with a special stamp. But first-class rates for business users - which account for the bulk of mail volume - would have been boosted 3 cents to 16 cents per ounce.

The commission majority apparently agreed with opponents of the citizen stamp, primarily the U.S. Chamber of Commerce, which charged that two first-class mail rates would amount to "discriminatory rate-making."

A spokesman for the business group said yesterday that although the citizen stamp idea "had a nice ring to it, when it comes down to the consumer they would be paying for [higher business mail] rates anyway."

The Postal Rate Commission could reverse its decision before a final announcement scheduled May 12, but congressional and industry sources said they believe the citizen stamp is dead and that 15-cents stamps soon will be ordered for all first-class mail.

Rejection of the two prices for first-class mail would represent the first substantive rejection of a Postal Service rate paln by the postal commission, which has been prodded by some members of Congress to establish a more independent course.

Governors of the Postal Service must ratify or reject the rate commission's decision, and a board of governors meeting has been scheduled for May 19 to begin work on the rate case.

Because every day of delayed postage increases amounts to huge revenue losses for the financially strapped Postal Service, government sources said it is unlikely that the board of governors would reject a commission decision. But the rate order could be accepted "under protest," requiring additional consideration of the citizen stamp while the 15-cent rate becomes effective on an interim basis.

Postal sources said the new rates would start either May 28 or June 4, both Sundays.

A Postal Service spokesman said that since no final decision will be evident before mid-May, there could be shortages of 15-cent stamps if that rate prevails. But there will be "lots of 13-cent and 2-cent stamps," he added. Oliver Wendell Holmes, a Supreme Court justice for 29 years, is pictured on the current 15-cent stamp.

The rate commission decision could compound the Postal Service's financial problems because the reduction to 15 cents from 16 cents in rates for business users would cut projected revenues by some $500 million - requiring upward adjustments in other rates that could force some customers to use alternative delivery systems.

Postal Service expenses are expected to increase later this year, when new contracts are signed with labor unions representing 570,000 workers, whose current agreements expire July 20.

The House has passed legislation to increase postal subsidies and restore some presidental and congressional control over the agency, but the administration opposes the measure and Senate approval is in doubt.