The latest chapter in the unfolding story of travel rewards programs may well be entitled, "Buy and fly."

Over the past 25 years, the majority of frequent flier miles have been earned for actually flying, followed closely by miles accumulated from buying with program-linked credit cards.

While the airlines decline specifics, evidence suggests a third category of mileage-earning is growing faster than either flying or credit card use, and could eventually become the programs' No. 1 generator of frequent flier miles.

A mileage mall is a network of online retailers which awards miles-for-purchases to members of a particular program.

Over the past few years, the largest U.S. airline programs have developed such online malls, with as many as 200 participating online stores, enabling members to earn miles for every type of merchandise.

American's AAdvantage eShopping mall, for example, allows members to earn one to three miles per $1 spent, at more than 200 online merchants, from the Apple Store to Nordstrom to Zappos.com.

Delta's SkyMiles Shopping Mall includes about 150 online stores, United's Mileage Plus Mall features more than 120 merchants, and so on.

Even smaller programs hosted by Alaska Airlines and Midwest Airlines have mile malls, although Alaska's directory lists only about 100 participating retailers, and Midwest's has just one-third that many.

Among the major hotel chains, Marriott and InterContinental have malls, with the Marriott Rewards and Priority Club Rewards Shopping malls each featuring almost 200 retailers. And Trip Rewards, the frequent-stay program for the Cendant Hotel Group (Howard Johnson, Ramada, etc.), offers points-for-purchases at a modest collection of 17 merchants.

There's also a mileage mall in the future of the American Express Membership Rewards program.

As part of their redesign, beginning Oct. 1, members will no longer earn double points for purchases charged to an American Express card at stand-alone supermarkets, gas stations, drugstores, the Postal Service, and wireless phone bill payments. Replacing the everyday double points offer will be an online mall where Membership Rewards members can earn two points per $1 spent at more than 100 retailers.

READ SMALL PRINT

As many would-be mileage collectors have discovered, it's not enough to be a member of a program and shop at a program partner's Web site. Miles and points are only registered and awarded when program participants begin shopping at the mall's own home page which generally can be found one or two clicks away from the Web site of the airline or hotel which hosts the program. From there, shoppers and their membership numbers can be tracked and any purchases made at participating merchants' Web sites can be credited to their accounts.

No miles are awarded for shopping directly on the store's Web site; there's no provision for capturing the shopper's frequent traveler information — and the programs won't award credit retroactively for shopping trips which bypass the mileage mall's main entrance.

MORE MILES PER DOLLAR

Many programs now have special "back to school" bonuses in effect, allowing shoppers to earn extra miles at selected merchants.

Through Sept. 17, AAdvantage members earn double miles at merchants participating in the AAdvantage eShopping mall. Members earn six miles per $1 spent at Target.com, Lands' End, and Overstock.com; four miles at Staples; and two miles at HP Home Store.

Members of Continental's program can earn five miles per $1 in purchases at Office Max.com through Thursday at the ShopOnePass mall.

Through Sept. 15, Northwest WorldPerks members who spend $150 shopping at the WorldPerks Mall will earn a one-time 500-mile bonus.

And through Thursday, double points are on offer at the Marriott and InterContinental malls. At the Marriott Rewards Mall, members earn double points for purchases at HP Home Store, Nordstrom, Over stock.com, and Staples. Shoppers at the Priority Club Rewards mall will double their points when buying from HP Home Store, Irv's Luggage Warehouse, FinishLine, Office Max.com, Overstock.com and Target.com.

SEARCH FOR REWARDS

Since there's no extra charge to earn miles for purchases at participating online retailers, and many brand-name retailers are affiliated with one or more mileage programs, ever-growing numbers of savvy mileage collectors make it a point to channel as much of their business as possible to merchants that award miles.

But with so many merchants offering miles and points in so many programs, it can be a challenge determining which retailers are linked to which programs.

And as new merchants are added to existing programs and new malls are developed by programs which previously lacked them, the difficulty increases exponentially.

Two Web sites, evreward .com and RewardsDB.com, simplify the connecting retailers and programs by allowing users to search for travel rewards opportunities by merchant name, category or program affiliation.

Want to earn miles for your next online book purchase? A quick search on either site shows that Amazon.com doesn't award miles or points in any travel-rewards programs, but Barnes & Noble offers three miles per $1 spent in American's program, five miles in Continental's program, and two miles in United's program.

The choice is clear: just buy, or buy and fly.

Reach Tim Winship at questions@frequentflier.com