Undaunted by President Carter's call for a curb in credit-card spending, the plastic money that sustains much of the fashion business, American retailers started yesterday in Milan on Round One of a three-week marathon of fashion shows and buying appointments that will move on to London by the weekend, then Paris.

Right on schedule, forsythia bloomed on the fairgrounds, where more than 10 manufacturers are showing wares for next fall. And since clothes and accessories are numero uno in the Italian balance of payments (followed by machinery, then trains), the organizers of this week's promotion are making sure the shows run on time.

While some stores have cut back on the size of their buying contingent, there are just as many American stores represented as last year. "How much they will spend we will only know after the week is over," explained Beppe Modenese (organizer of Promozione Moda Italiana (Italian fashion promotion).

For sure they will spend more. As buyers have found out already, prices are up as much as 30 percent, with 15 percent considered an average.

The only prices that don't seem to be up are the Italian leather clothes, which took a monumental price leap a season back. A tunic and pants outfit in caramel-colored suede shown yesterday by Genny, for example, was estimated to retail at $5,000.

While stores insist that their wealthiest customers will continue to buy quality items, price is very much on the minds of everyone here. "Don't buy anything unless it is irresistible," Ellin Saltzman, fashion director of Saks Fifth Avenue, has told her buyers. "No one is going to be willing to spend money for more of something she already has in the closet," says Saltzman, who takes the same line toward purchases in New York and other fashion centers.

One collection several American stores have found irresistible is Missoni, master knitwear makers, whose prices are up 30 percent. "In spite of the very high price they have sold well immediately," says Benita Downing of Neiman-Marcus. "And while the prices for fall are much higher still, the colors and textures will be hard to resist." Downing and others put Missoni's strongest suit as the reversible coats, tent shapes, sweater coats, chemise dresses and full skirts in mixed patterns, stripes, and tweeds particularly.

Price increases are not only on the runway here, Italians are dealing with a 21.7 percent annual inflation-rate which, according to Elisai Massie, an economist, has cut back spending for the last two years. "But eventually," Massie says, "people simply have to buy new clothes and that seems to be happening now."

The Italians call it "l'arte di arranglarsi," the art of making do.

Jack Schultz, executive vice president at Bloomingdale's, is not so philosophic about the economic problems at home, particularly Carter's suggestions to inhibit credit-card usage. (At Bloomingdale's more than half the business is done on credit cards). President Carter's suggestion of higher charges on monthly statements, stiffer repayment schedules, reduction or even elimination of the grace period before interest is charged and a tougher policy on delinquent payments, astonishes Schultz.

"Isn't the freedom to buy one of the great American rights?" asks Schultz, who adds, smiling, "like motherhood."

"I guess we will have to hire a lot more lawyers to figure out how to conform," he said, adding philosophically to himself, "and then we will."