One last stop on the itinerary of many international travelers, just before they board the flight home, is a visit to the airport's duty-free shop. It's a convenient place to buy a last-minute gift of perfume or a bottle of fancy cognac or whiskey, often -- but not always -- at a nice savings over prices elsewhere.

For example, a quarter-ounce bottle of Joy de Jean Patou, one of France's most prestigious perfumes, sells for about $60 at Charles de Gaulle Airport. A Washington department store prices the same perfume at $75, plus an additional $4.50 in sales tax. That's a saving of $19.50 a bottle -- not too bad, especially if you are buying more than one as special gifts.

Paralleling the tremendous growth of international air travel since the 1950s, duty-free shopping has become a $5-billion-a-year business at some 300 airport duty-free shops worldwide, including about 30 shops at airports in the United States.

Both Baltimore/Washington and Washington/Dulles international airports have duty-free shops. Shops can also be found on many cruise ships, and duty-free goods often are sold aboard airplanes on international flights, though in-flight choices usually are quite limited.

Traditionally, the fastest-selling items -- and the best buys -- are liquor, French perfumes and tobacco products. But increasingly, many duty-free shops are adding designer leather and silk goods to their shelves, as well as other luxury items such as watches, jewelry, cameras and even gourmet foods and chocolates.

The Washington-based International Association of Airport Duty-Free Stores, representing airport duty-free shops in North and South America, estimates that France's leading producers of fragrances "sell more than 30 percent of their lines in duty-free stores worldwide."

Given the variety of merchandise available in duty-free shops, some buys understandably are better than others. But not everything is necessarily a bargain. Some merchandise may be no cheaper -- and it may even be more expensive -- than what you can get it for in other stores. These items generally are available in duty-free shops simply as a convenience to travelers.

Only travelers immediately departing a country can make purchases in a duty-free shop. Here are a few guidelines for successful shopping:

You should, of course, be aware of what "duty-free" means.

Many American travelers, particularly those going abroad for the first time, are confused by the term, says Susan Stackhouse, director of communications for the International Association of Airport Duty-Free Stores.

A frequent misconception, she says, is that purchases made in a duty-free shop abroad are exempt from duties and taxes collected by the U.S. Customs Service when you return. They, in fact, are not exempt (except as part of the standard $400 exemption granted to all U.S. residents).

Duty-free shops cut prices by stocking merchandise that normally is heavily taxed (liquor, for example) when sold on the regular domestic market. But international passengers can purchase these items without paying such locally imposed taxes. They are thus free of duty in the country where the purchase is made.

The items, nevertheless, are still subject to any applicable U.S. duties and taxes. U.S. residents are entitled to bring home up to $400 in purchases free of federal taxes, but anything bought in a duty-free shop must be added to the total amount of purchases. If you go over the $400 exemption, your "duty-free" purchases may have a U.S. duty imposed on them.

Savvy shoppers understand that the best way to spot a bargain is to know what you want and how much it customarily costs.

That, of course, requires a bit of advance planning, something Stackhouse advocates. She suggests that before American travelers visit a duty-free shop, they do some before-departure comparison shopping to see how much the items they are interested in cost in the stores at home.

For example, cameras and other photo equipment frequently can be purchased at big-city discount marts for much less than you might pay at a duty-free shop, says Stackhouse. And for expensive purchases, you might prefer a less-hurried atmosphere than an airport.

Even duty-free shops may undercut each other. The same $60 bottle of Joy available at the De Gaulle airport outlet currently is offered for only $49 on Trans World Airlines international flights, including TWA's flights out of De Gaulle.

Stackhouse notes that most duty-free shops stock name-brand merchandise, which makes comparison shopping fairly easy. But be sure you have the correct model number and size. And be particularly careful in comparing liquor prices. Most duty-free shops sell liter bottles, which contain 33.8 ounces. A standard "fifth" sold in U.S. liquor stores is really fourth-fifths of a quart, or only 25.6 ounces.

Anyone contemplating substantial purchases at a duty-free shop should allow plenty of time at the airport. Shops can become quite busy in the hour before one or more wide-bodied jets are scheduled to depart.

Even when bargains are few, duty-free shops can offer the value of convenience, particularly for travelers who are reluctant to spend their limited sightseeing time abroad searching for gifts.

The typical vacationer usually has a friend or relative at home to be remembered; and if you haven't found the time to buy something by departure day, there's that one more chance at the airport. Perhaps as important, any gift purchased at the airport is something you haven't had to tote for several days.

Another advantage of duty-free shopping is that you avoid the hassle of applying for a refund of the Value Added Tax (VAT), a European form of sales tax imposed on most consumer items. Getting the refund usually requires paperwork and, perhaps, a long wait in line at the airport. No VAT is charged on purchases in a duty-free shop.

Many travelers look on the duty-free shop as a place to spend any remaining foreign currency in their pockets, perhaps buying a bottle of their favorite whiskey, usually at a savings of at least a few dollars. U.S. residents 21 and older are entitled to bring home one liter of alcohol free of U.S. taxes and duties. (Any liquor you buy, however, counts as part of the $400 in purchases you are allowed to bring home duty free.)

A liter of 12-year-old Chivas Regal scotch whiskey, costing no more than $18 in duty-free shops, sells for about $20.50 in Washington, plus another $1.23 in city sales taxes.

Stackhouse's organization caters mostly to foreign travelers visiting the United States and other countries in this hemisphere. The Japanese, particularly, are avid duty-free shoppers, she says, who have made the duty-free shops at the Honolulu and Anchorage airports the busiest in the world.

But her association is also trying to interest Americans going abroad in shopping at airport duty-free outlets in the United States prior to their departure. Stackhouse suggests buying business and social gifts abroad or tobacco and alcohol to be consumed on your trip. Anything you buy in a U.S. duty-free shop, however, is subject to duty if you bring it back to the United States.

You save a few dollars on what you might pay in your neighborhood store for liquor and cigarettes and perhaps quite a lot over prices charged in a foreign city.

One caution: Countries place varying limits on the amount of liquor and tobacco that can be imported for gifts or personal use. U.S. duty-free shops should be able to advise you on these regulations.

The BWI duty-free shop (301-859-8507) is open 90 minutes before international flights; the shop at Washington/Dulles (703-661-8815) is open daily from 9:30 a.m. to 8:30 p.m.

The U.S. Customs Service publishes an informative booklet on U.S. customs regulations, "Know Before You Go." Free copies are available by sending a postcard to U.S. Customs, P.O. Box 7407, Washington, D.C. 20044.

DOLLAR UPDATE: With international efforts underway to try to drive down the value of the U.S. dollar, travelers are faced with a problem of how to protect themselves against losses caused by a fluctuating exchange rate.

While the dollar was climbing steadily on exchange markets, as it did during much of the past three years, the best advice was to take U.S. dollar travelers checks abroad and use your credit cards.

Dollar checks were welcomed when the dollar was at its strongest, and they were easily exchanged at increasingly favorable rates. Charges made on a credit card one day might be processed at a more favorable rate a day or a week later.

But now a new strategy should be considered.

If the dollar continues to slide (which, of course, is not altogether certain), you will want to lock in current exchange rates before it drops any more. A way to do this is to buy travelers checks in the currency of the country you are visiting. Foreign-currency travelers checks are available in French francs, Swiss francs, German marks, British pounds and Japanese yen.

If the dollar is bouncing up and down, consider carrying both dollar and foreign-currency checks. On days when the dollar strengthens, exchange dollar checks. When the dollar sags, use the foreign-currency checks.

One advantage of foreign-denomination travelers checks is that you can cash them for the local currency almost anywhere. You avoid the necessity of searching out a bank or exchange bureau, where the lines have been long this year.

And when the dollar is fluctuating, and especially if it is declining, use caution with credit cards, especially for very large purchases. An item or expensive meal charged one day could become even more costly the next, since the exchange rate is calculated not when you make the purchase but when the paperwork is processed. That could be several days later, when the dollar is worth less.

BRITRAIL: In a move to attract more business travelers, BritRail is launching two programs geared toward the corporate man or woman on the move.

Kings Cross Station in London now has a lounge for use by first-class passengers, and before the end of the year lounges will be opening in other London stations. Designed to provide a comfortable and quiet environment for brief business meetings or a quick meal, the lounges include photocopiers as well as hotel reservation and car hire facilities.

For more elaborate business meetings, confidential discussions, small sales conferences or corporate lunches, an entire rail car can be rented. The car, which seats a maximum of eight around a conference table, can be attached to trains serving major cities. Basic cost of hiring the coach is around $425, and each passenger also must pay the first-class fare to the destination.

For more information: BritRail Travel International, 630 Third Ave., New York, N.Y. 10017. (212) 599-5400.