Business Notes MARKETING

American Airlines, which introduced frequent-flyer bonuses in 1981, created a mixed blessing for the airline industry. Customers have collected so many miles -- an estimated 70 billion in American's case -- that carriers have huge future obligations to give out free-ticket awards. In part to whittle down that liability, American Airlines has launched AAdvantage Extra, a system that allows passengers to exchange their unused mileage credits for discounts on everything from diamond jewelry to inflatable catamarans. Passengers who have flown at least 20,000 miles receive a catalog listing available rebates when they buy such items as a Toyota Tercel ($400 off for 20,000 miles) or a full- length lynx fur coat ($15,000 for 120,000 miles). Still, some travel- industry experts consider the barter system overly complex and many of the rebates less than breathtaking.