None of them include Southwest's fares, however, or fares on the smaller but growing Allegiant Airlines.

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And check out AllegiantAir.com to see if Allegiant Airlines flies where you're headed, since it, too, sells tickets only on its own website.

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Spirit airlines (joined with Allegiant and Southwest airlines) challenged the regulation, which had been upheld in the U.S. Court of Appeals last July.

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Allegiant joins Spirit Airlines as the second U.S. carrier to implement fees for carry-on luggage.

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Two U.S. airlines, Spirit and Allegiant, also charge passengers for carry-on baggage.

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Although it could also pave the way for further expansion by discount airlines such as Spirit Airlines Inc. and Allegiant Travel Co.

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These include AirTran Holdings, Allegiant Travel, Hawaiian Holdings, JetBlue and Southwest Airlines.

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An ultra-low-fare model gives carriers such as Allegiant and Spirit more flexibility in unbundling the costs of air travel, Cordle said, but other airlines will be watching the carry-on fee closely.

CNN: Allegiant Air adds carry-on fee