With the right kind of salesmanship, Americans have bought pet rocks, subdivision lots in the Arizona desert and dairy creamers blended by industrial chemists. But the question remains: Can they be sold the nine-digit zip code?

They can, believes Walter E. Duka.

Duka may be dreaming, but that's not what he gets paid for at the U.S. Postal Service, where he is assistant postmaster in charge of communications. It is Duka's job to sell America on the nine-digit zip code. In case you haven't heard, the Postal Service plans to assign an expanded code to every address by next October.

Has the Postal Service become another victim of the digital diarrhea that's been going around? No, Duda says, the agency needs all nine digits to get the most efficient use of a fancy new automated sorting system that, all told, will cost $887 million. With nine digits, he says, the system can sort mail all the way down to one side of one block, even one building or one floor in a building.

The problem is that there has to be widespread cooperation from the public, especially business, which does most of the mailing, to make the nine-digit zip code work. Toward that end, it is Duka's job to make the expanded code look like the best thing since . . . well, the five-digit code, which is used by 97 percent of all mailers.

There is ample evidence to suggest that Duka has a herculean task. On his desk is a pile of clippings of some initial responses to the Postal Service's plan, and what they show is that all around America editorialists have been harrumphing about the numerical expansion. A recent editorial in the Honolulu Advertiser is typical: "In an increasingly desperate world, the Postal Service wants to drive the final nail in the coffin."

It does not take much imagination to picture what a cartoonist could do with the subject, especially if he had to queue up recently at his neighborhood post office to buy a 15-cent stamp.

Duka concedes most of the press reaction to the nine-digit zip code has been negative.

He also acknowledges that 122 members of Congress have urged Postmaster General William F. Bolger to hold off implementation until the "social impact" is considered and the technical details of the change get closer evaluation. And, too, there was the Senate's vote Thursday to prohibit the Postal Service from using any of its subsidy for the new zip, not at least until June 1.

He further acknowledges that the American Postal Workers Union, which represents 250,000 of the service's 660,000 employes (and doesn't want to lose any members), agrees with the congressional critics, and, further again, that many businesses and mailing groups are less than thrilled because of the tremendous costs of converting their 8 billion address listings.

But wait, there is another side to the story, Duka argues. Choosing his words carefully so as not to further irritate any of the Postal Service's many critics, Duka ticks off his points:

Hardly anyone will get a whole new set of nine numbers. Except in a few cases, addresses will retain their five digits and be given four new ones. The old and new numbers will be joined by a hyphen, apparently so they can be digested in two smaller mental bites rather than one big one.

No one will be required to use the expanded number. "It's strictly voluntary," Duka says. (Spoilsport critics point out, though, a recent mailers' guide issued by the Postal Service, which says: "When you fail to use a zip code, your letter is sorted to a special bin . . . . As a result your letter may miss a special dispatch.")

Some key members of Congress are supportive of the expanded zip code. Sen. John Glenn (D-Ohio), chairman of the Senate Governmental Operations subcommittee with postal jurisdiction, is among them, as are Reps. James M. Hanley (D-N.Y.), chairman of the House Post Office and Civil Service Committee, and Edward J. Derwinski (R-Ill.), its ranking minority member. c

There has been some favorable editorial comment. "Critics should back off from their knee-jerk reaction," said the Tacoma, Wash., News-Tribune. And while there was an editorial in the Los Angeles Times titled "Zapping Your Mail," there was another in the Houston Post headed "More Zip in the Mail."

The dissenting editorialists, while in the majority, may not be representing any significant section of the public. A survey commissioned by the Postal Service indicates there will be wide acceptance of the nine-digit code. Of the thousand people surveyed in all sections of the country, 79 percent said they were likely to use it when they send letters.

(The Postal Service, however, hasn't cited this poll in any of the congressional testimony of its officials, perhaps because only 1,007 people were interviewed, and they were average members of the public, who account for only 7 percent of the 105 billion pieces of mail that go in and out of post offices annually.)

Business, which will suffer a hardship in making the costly conversion, will get "incentive" assistance from the Postal Service, assuming such aid is approved by the independent Postal Rate Commission.

The expanded zip code, together with the new automated system of sorting, will result in $1,156,375 in manpower savings through 1993, assuming there is gradual acceptance rising ultimately to 90 percent. These savings, Duka says, will mean fewer rate increases than would otherwise occur.

But even as Duka is talking about increased efficiency, the Postal Service is asking for an overall 28 percent increase in postal rates. With a weary voice, Duka admits, "Yes, the timing is unfortunate."

But he is not so resigned when critics argue that the Postal Service, based on earlier fiascos involving its attempts to automate, should get close scrutiny when it proposes to spend nearly a billion dollars on a plan that involves machines being able to "read" millions of letter addresses daily and then sorting the mail into a neat -- and accurate -- package for the delivery person. The critics point to the Postal Service's disastrous attempt to highly mechanize 21 bulk mail centers.

"That's mixing apples and oranges," he says. "New facilities had to be built for the bulk mail centers," he said, "and they were predicated on an independent transportation system that never developed. The new automated system is being added to existing facilities, and the equipment is already in use in other countries."

He also says that the Postal Service, just to make sure about the technology, ran dry runs using address-reading machines on 30 million pieces of mail, and there were no significant snafus.

Duka saves his best line for last: "The conversion to a nine-digit zip code, if widely accepted, will save 16,000 manhours annually. We are the only agency that's trying to diminish itself rather than expanding."

Ronald Reagan and the new 97th Congress, are you listening?