Spirit Air, a small low-cost airline headquartered in Miramar, Florida, will soon be the first airline to charge for carry-on bags. Fees for luggage put in overhead bins could climb as high as $45, while small items that fit under the seat would remain free. Spirit Air says the new charges will help it lower base airfares. "Bring less; pay less. It's simple," a Spirit exec said. At the gate, carry-ons will cost $45; if paid for ahead of time, $30; and for frequent fliers the price will be $20. Airlines are increasingly relying on fees to boost revenue.