A $300,000 collection of Peter Max's cosmic flower children, the commercial art rage of the '60s, now decorates the halls, offices and warehouses of the General Services Administration because the U.S. Customs Service rejected it.

But the new GSA administrato, Joel W. (Jay) Solomon, is making a renewed effort to have the 200 weather-resistant posters put up at Canadian and Mexican border stations. A previous administrator, Arthur F. Sampson, had commissioned the series - after having seen a Peter Max television advertisement for Seven-Up - to represent America to immigrants and tourists.

Customs officials, however, found the floaty, smiley figures "inappropriate" and felt that they "presented a poor first impression to people coming to the United States," according to a spokesman for Robert Chasen, the new commissioner of Customs, who is on vacation.

Max's work has often been linked with the drug-oriented youth culture of the '60s and, of course, the Customs Service is mandated to stop drug traffic. Max himself says he had "experimented" with LSD, but that "the peace I found through Yoga" was more of an influence on his work.

"Drugs were clearly the primary reason of the Customs' concern," said Kent Slepica, the former director of special projects for GSA who was responsible for the welcome signs. Slepica said that, "Customs felt the signs were contrary to the image they wanted to portray about drug control. They were of the opinion that it signified drugs - psychedelic colors and images became synonymous with drugs during the '60s.

Solomon and Roland Raymond, the assistant commissioner of operations for Customs, both talk about the controversy in terms of acceptance or rejection of "modern art."

"Most of the time modern art is hard for local people to accept," said Solomon, who sees the pictures as conveying "happiness and cheerfulness."

He continued, "It's the art of our times. If it's selected by people who know what good art is, we should accept it."

Raymond called it "modern art, surrealistic art, which is not really for the uninformed traveler."

Actually, Max has had an almost unprecedented mass acceptance. In the mid-60s, his designs could be seen everywhere - in the Beatles' movie "Yellow Submarine," television, on buses, sheets, stationery, clothing, plastic pillows, tea bags, cosmetics, clocks, household appliances, books buttons and dishes. His 10-cent U.S. postage stamp was reissued three times.

Max said he has been on a "four-year retreat" until a few weeks ago, doing only paintings, including eight of an 18-mural series commemorating Jimmy Carter's inauguration. He calls Carter a friend - the President attended an opening of Max's in a Georgia gallery - and now hopes to get federal work "using color to spread optimism throughout the greatest country in the world, the new Mecca, the only country that has a chance of bringing peace to the world."

Solomon agrees that the Max murals could cheer up immigrants. But he has had no luck, so far, in convincing Chasen, who wrote Solomon that he had "reviewed" the decision of his predecessor and would not change it.

Solomon plans to request a meeting with Chasen, and said he could bring other influences to bear, including the enthusiasm of Mexican and Canadian officials whose countries the pictures would face.

There are seven Max paintings, made, with some technical alteratins from which the 200 murals were made, with some technical alterations - masses of lavender in the paintings were changed to blue, the example. They were to be hung above bilingual welcoming signs, which the Customs Service was willing to accept without the art work. At the time the Max work was commissioned, it was intended to be part of GSA's contribution to the Bicentennial.

Solomon believes that he can convince Customs to accept and hang the collection, "in keeping with President Carter's policy - let's use what we've got."

However, he has a list of alternatives prepared in a GSA memorandum:

"1. Offer the paintings to the National Gallery of Art and the National Collection of Fine Arts.

"2. Auction the paintings off at a public auction.

"3. Create a traveling exhibit to various cities.

"4. Place the paintings in a new federal building which did not already have artwork.

"5. Display the paintings in GSA central office."