IT'S MIDDAY at Honolulu Airport, and a late-arriving jumbo from the Far East has just disgorged its lot of soiled and spent passengers. Two returning U.S. citizens claim an odd-lot assortment of bundles plus serveral suitcases and wearily make their way to the United States Customs counter.

Yes, says the man, they're over the duty-free allowance and wish to make a declaration. His words are stright-forward but his anxious tone adds an extra meaning, and his wife's face mirrors his voice. They know this is going to cost them. They don't know how much.

The Customs officer looks at their list, then their purchases. They've picked up a bit more than $500 worth of souvenirs.

"You don't owe anthing," says the inspector. "I guess you didn't read the notices. You can bring in $300 worth of goods each. They changed the duty-free rules last November, you know."

Apparently many trvalers don't know, although Customs has made considerable effort to let them in on the news, including details on the constantly lengthening list of products, thousands long, that can come in duty-free in any amount. Since spring, Jonathan Winters has been comically passing the word in public-service TV announcements, and Arte Johnson has been doing the same on radio.

Customs has also been contacting the nation's travel agents, asking that they not only alert their clients but also point ot that they can get a copy of all the rules in a pamphlet called "Know Before You Go." It's free for the asking from U.S. Customs Public Affairs, Washington, D.C. 20229.

There is, though, one other piece of news to know before you go. It can be summed up in a single sentence: If you're after "bargains" abroad, consider that a lot more has changed besides Customs regulations and that some once-sweet shopping tips have gone sour. For instance:

"Hong Kong is hog heaven for anyone who wants to pick up tailor-made clothes at peanut prices."

"A long time ago, yes. But that was before the ready-made garment industry got going and grabbed off most of the expert workers and before inflation further pushed up prices. Now lots of Hong Kong residents get their custom clothes in the Philippines or Korea.

"Load up on leather goods in Spain."

In the 1960s this was a great idea, but it didn't survive the '70s. Well-made handbags and clothing in particular are now as pricey in Madrid as they are in London and Paris, and that is very pricey.

"Athens is the place to get your hands on bargain-priced furs."

By all means handle them. Smell them, too. Many fur coats and jackets sold here are not-too-well-processed bits and pieces stitched together patchwork-quilt style, so don't assume you've suddenly discovered "quality" skins at fantastic markdowns.

Denmark has built up a very competitive business in seal and Persian lamb, but for mink, even many Europeans look to the U.S. for value and price. For nutria, there's still Argentina, though processing there has lately drawn some questions. Of course, buying furs abroad you also need to check with Customs for what's on the endangered species list. Canadian seal coats, for instance, are prohibited and will be instantly confiscated if you try to bring them here.

"Buy emeralds in Colombia, and the savings will pay for your whole trip."

First of all, understand that prices for the good stuff have soared world-wide. Therefore you're talking about big money -- everywhere. Second, realize that most people just aren't knowledgeable enough about gems to bring off a coup, and they wind up with ordinary stones at extraordinary prices -- in other words, a pleasant enough souvenier but nothing the Smithsonian is likely to cast envious eyes upon.

Buying with resale in mind is really a pipe dream because it's hard to find a private buyer, and jewelers are unlikely to reward you with retail rates for something they can get wholesale. In buying just for your own enjoyment -- anywhere -- bear in mind, too, that the "profits" on unset stones tend to disappear when you go to have them set, and that the quality of settings in many gem-center countries is often unsophisticated if not just plain poor.

"Antiques are the thing to buy in India."

Yes, they might be. But taking anything more than 100 years old out of the country is a no-no these days. What you see on sale, therefore, is usually not anything as old as it may appear. The Instant Age Industry flourishes in India; bronzes are often buried until "greened," paintings literally smoked. Reputable dealers will of course say what's what, but there's no shortage of not-so-reputable ones who just mumble and play along with your assumptions.

"When in London, stock up on china."

If you haven't looked at china prices lately, you're in for a shock, in this or any other country. Even in London, a five-piece place setting of Coalport's Indian tree, for instance, sells for roughly $75. However, that is still less than what's asked here, which is $110 or thereabouts. But other figures need looking at, too; there the fee for mailing home what you can't carry and, in the United States, the markdown price at the big department store annual spring and fall chinaware sales. In the end, by shopping at a sale, you're likely to do as well on English china in this country as you would in London.

"Of course, everything is lots cheaper at airport duty-free shops."

Don't bet real money on it. Duty-free shops vary like crazy and these days frequently "promise" more than they deliver, even on one-time standard savers like liquor and tobacco. A few still resemble the bargain hunter's friend in some departments, however. For instance, that Coalport Indian Tree place setting is available at Shannon Airport's duty-free shop for $45.25. Shannon also has a 1-ounce bottle of Joy perfume for $72.50, whereas it's about $115 at Vienna Airport. Similarly, in Vienna it's $36 for the same small flacon of Joy that sells for $26 at Tel Aviv Airport. (In the U.S. 1 ounce is about $130 plus sales tax, and the flacon, with one-fifth of an ounce, is around $38.)

"Sun seekers are lucky because you really clean up shoppin in the Caribbean."

The Caribbean is a large area, and not all its islands were bonanza lands even in the old days. Still, there were and are notable stretches -- st. Thomas and St. Croix in particular -- but it's sad to note that the gap between prices there and prices here has been closing. More important, though, is probably the face that the big bargains were always in luxury goods, and escalating prices on such items everywhere mean that $92-an-ounce Joy in St. Thomas is just as out of reach to the moderate-income traveler as $130-an-ounce Joy in the U.S.

But never fear, salvation is near. And you really can't keep a good shopaholic down. Those who've been priced out of one "bargain" can be counted on to ferret out another. Don't tell a soul, of course, but they've got some fine buys on Dijon mustard in Paris and the price of cashew nuts in Sri Lanka makes them a real steal.