It's one thing to buy your ticket for a flight, but if you want to select your seat, you may have to pay an extra travel fee. Many airlines, such as AirTran and Spirit, charge you to select the seat you want. At AirTran, the cost ranges from $6 to $20 per ticket.
Even if your airline doesn't charge for seat assignments, you may have to pay if you want to sit in a row that has extra legroom. At Spirit, the fees for seats with more leg space start at $12 if reserved in advance.