Allegiant Airlines is a small carrier headquartered in Las Vegas. They fly direct routes from smaller airports to Las Vegas, Orlando, and now Hawaii, and a few other cities.
They are very profitable because their schedules are limited: from a given airport, they will have only two or three flights a week, and because people don't have to depend on them like they do more traditional airline service, the schedule can be changed when the number of passengers declines. If a competing airline moves in on a given route, Allegiant just pulls out all together.
Another source of profit for Allegiant is all the fees they tack on. They were one of the first airlines to charge for just about anything you can think of, and many of the other carriers have followed suit. You'll recognize some of these, and maybe not others, yet.
Seat selection fee: $10 - $75
Priority boarding fee: $5 (I'm not sure why anyone would get this if they'd paid the seat selection fee)
Trip flex fee: $11 - to avoid change or cancellation fees
Food or beverage: $1.99 to $12.99
Pet in cabin: $100
Checked baggage fee: They charge different amounts for checked bags, according to the route. It ranges from $14.99 to $35 per bag if you pay when you book your ticket, $35 at check-in if you didn't pre-book it, $50 for the third bag. (This is old news now that all the airlines do it.)
Carry-on baggage fee: $10 - $35. Again, this depends on the where you are flying from/to. And if you don't pre-book it, it's $35 at the airport. The flight I usually take, from Colorado to Las Vegas would cost $13 for a carry-on if I pre-booked it. The only 'free' carryon is your purse or laptop or a food container that can fit completely under the seat in front of you, in other words, what airlines refer to as your 'personal item.'
Carrier Usage Charge: $10 for booking online or through their call center. The only way to avoid this is to get to the airport in the 45 minutes on the one or two days a week that they are selling tickets. They do not sell tickets in person at any other time.
Call Center Booking fee: $14.99 for booking through their call center IN ADDITION to the $10 Carrier Usage Charge. So it costs $24.99 to call and make a reservation.
Their latest is to charge $4 per segment for the use of a credit card as opposed to a debit card, so that is $8 for a round-trip.
They decided to do this because last year federal regulations changed so debit card fees to merchants are lower than credit card fees to merchants. Only 30% of Allegiant's customers were paying with debit cards, so they are trying to steer people to use them more.
I don't have a debit card, and personally don't like them because of the ease with which someone can steal the number and drain the bank account with which the card is associated. If I had one, I would want to have a separate checking account for it, and that would be a hassle to keep track of in addition to my other checking accounts.
Allegiant rationalizes all their add-on fees by saying other airlines build all this into their fares, and I think they do, to a certain extent. Allegiant says it only charges you for what you want to use.
Their customers don't like all the fees, but the convenience of being able to get direct flights out of small regional airports keeps them coming back and the Allegiant flights are always full.