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Minneapolis-based Sun Country Airlines charges a $44, round-trip booking fee online, something the carrier has done for a year and a half but customers only recently discovered this week.

Some passengers thought this was a lack of transparency from the airline, though it's the same fee other low-cost carriers had already implemented. What's more, the company said it's also baked into the advertised price — no surprise add-ons at checkout — but Sun Country just doesn't explicitly break out the applicable taxes and fees from the base fare during the booking process, as other carriers do.

Kyle Potter, editor of Thrifty Traveler, discovered and wrote about Sun Country's "passenger interface fee" — charged since April 2022 — after a recent reader tip. Sun Country Airlines Chief Marketing Officer Brian Davis maintained the leisure carrier's listed fare hadn't change because of the fee.

"Any price you see advertised includes this passenger interface fee along with other taxes," Davis said.

To find the fee, passengers have to go to the "new bag fees and optional services" hyperlink at the top of Sun Country's home page under the booking windows, then scroll to "passenger interface charge" to find the fee description.

Charging a passenger interface fee for online or phone bookings is a way for airlines to avoid federal excise taxes — and thus boost profit — as long as the carriers give passengers a way to avoid the fee. To avoid the charge on one ways or round trips, passengers can go to to the airport and buy directly from a ticket agent.

Major carriers, such as Delta Air Lines, don't tack on these fees. According to Potter, they are routine for low-cost carriers. Spirit has a "passenger usage charge" of nearly $23 per segment (from one airport to another), while Frontier calls an identical amount a "carrier interface charge," he said.

"Sun Country is far from the first airline to charge people this kind of fee," Potter said. "The biggest takeaway for consumers should be less about Sun Country and more that these are the kinds of games that [budget] airlines across the country play."

Even at other carriers that detail the fee, passengers might not understand what it entails.

"Who knows what 'passenger interface charge' means, except for the very few nerds like me that know that other airlines uses some variation of this terminology to say, 'Hey, if you book your tickets online, we're going to charge you for it,'" Potter said.

The fee didn't result in a lift in Sun Country prices for the most recent quarter. Sun Country's average fare per seat for the third quarter ending Sept. 30 was down by 9% to $153 from the year-ago period.

Growing Sun Country calls itself a new breed of hybrid low-cost air carrier, flying scheduled passenger service, charter flights and cargo routes focused on Amazon. The carrier flies throughout the United States and to destinations in Mexico, Central America, Canada and the Caribbean.