Think bank statements, neighborhood newsletters, airline frequent flier program updates and targeted advertisements from stores where you actually shop.

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Most prominently, SAS Airlines has, since March, been inviting the 50, 000 members of its EuroBonus frequent flier program to use NFC-enabled readers at its gates at airports in Scandinavia for flights within the region.

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Customers would also have to be informed how many seats were available for purchase with frequent-flier miles -- carriers haven't made it easy to compare one frequent-flier program with another.

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Frequent-flier program experts say airlines may simply be worried about losing revenue to other websites.

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The AirTran promotion, for instance, requires members of its frequent-flier program to register for the promotion before the flight departs.

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