Passengers who don't pay in advance for a carry-on bag will be hit hard financially at the airport gate.

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Spirit Airlines will punish procrastinators by charging $100 for travelers who wait to pay for their carry-on bags until they get to the gate.

The higher charge will begin Nov. 6, Spirit has announced . The fee has been $45.

Spirit and Allegiant are the only airlines in the U.S. that charge for carry-on bags.

Spirit’s rationale? It aims to make customers so wary of the steep fee that all will pay ahead of time. However, Spirit is also raising other bag fees effective Nov. 6. A carry-on bag paid for at booking will be $35, up from $30; a carry-on bag paid for at the airport check-in counter will be $50, up from $40.

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That means that a family of four with only carry-on bags will pay between $140 and $400 each way.

Spirit also is raising prices for checked bags. A first checked bag paid for during online booking will be $30, up from $28. At the airport counter, it will be $45, up from $38, according to Spirit’s website.

All these moves are an extension of the ultra-low-fare, bare-bones carrier reputation that the Miramar, Fla.-based airline embraces. Its business model is based on charging low ticket fares but adding on fees and charges for reserved seats and baggage. Known for its minimal customer service and outspoken CEO, it unapologetically courts travelers who look only at ticket prices, not the extra fees. All those fees make up 40 percent of Spirit’s revenue.

The federal government has fined Spirit several times for violating price and customer service regulations. The most recent was in January, when the Department of Transportation fined the airline $100,000 for failing to keep track of or respond to complaints from disabled passengers. Last year, the DOT fined Spirit $50,000 for deceptive pricing on its advertising and tweets.

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