“There are multiple ways you can improve upon your boarding zone,” said Andy Jacobs, the president of a candy company who travels about twice a month. “As a diamond member on Delta, I never have a problem securing space.”

Many travelers may not realize it, but a seat ticket does not automatically entitle them to overhead space. Once space runs out, passengers must check their luggage at the gate, without paying a fee — and then wait for it at the baggage claim at their destination. Airlines are capitalizing on the fact that many fliers are willing to pay for carry-on convenience.

When Mr. Jacobs is not flying Delta, he relies on credit cards to jump to the front of the boarding line. “Those cards are worth it,” he said. “I actually carry three different airline affinity credit cards.”

He has good reason to get to the front. “As a chocolate salesperson, I need to bring my bags on the plane so the chocolate won’t melt,” he said. “When you’re flying to a major customer and you pick up your bags at baggage claim and your samples are melted, that becomes a pretty big problem.”

As Mr. Jacobs has learned, there is only so much room.

And there is little chance that the free-for-all at check-in will ease soon. Airlines rely increasingly on fees, experts say. In 2012, domestic carriers collectively earned roughly $3.5 billion in checked-bag fees, up from less than half a billion dollars five years earlier, according to the Department of Transportation. Although exact figures are not yet available for early boarding revenue, analysts say it is increasing.