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For an airline that proudly touts its bags- fly-free policy, Southwest Airlines is, like other carriers, finding ways to make more money through fees just the same.

The airline now offers customers the opportunity to purchase a spot in the coveted "A" boarding group. The cost is $40, and the spot can be bought only at the gate the day of travel 45 minutes before departure.

Southwest is known for its open seating in which customers are divided into groups A, B or C. Beyond seat selection, boarding early can also assure room in the overhead for carry-on bags.

The airline said it tested this new boarding option in San Diego last month and received positive feedback.

Southwest already has Early Bird Check-In, which automatically checks customers into their flights more than 24 hours in advance. The cost is $10, but it increases to $12.50 next month. The airline said people who opt for early check- in "more than likely" will receive an "A" boarding pass, but that's not guaranteed. Southwest also offers a Business Select Fare, which guarantees an A1 - A15 boarding pass plus a coupon for a drink onboard the flight.

Other airlines also have fee-based priority boarding options. Last month, American Airlines added Choice Essential and Choice Plus fee packages, which, in addition to such other perks as no-change fees and a free checked bag, allows customers to board in Group 1. The round-trip cost is $68 and $88, respectively.

"At Southwest, we have developed boarding options that fit a variety of our customers' needs - from EarlyBird Check-In to our Business Select Fare," said Kevin Krone, Southwest Airlines vice president of marketing, sales and distribution, in a statement. "Offering customers the option to improve their boarding position on day of travel is one more way we can offer the travel experience that best fits their needs."

And, a new opportunity for the airline to make more money through fees.