The airline industry's increasing reliance on extra fees is spawning a sort of arms race between discount carriers and their thriftiest customers to outsmart each other.

Global airlines brought in $27 billion in nonticket revenue last year, an 11-fold increase over 2007, according to airline-consulting firm IdeaWorks Co. Nonticket revenue includes fees for extras, as well as sales of frequent-flier points, hotel rooms and rental cars, among other things.

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