Spirit Airlines said Tuesday it would post a wider-than-expected loss for the last quarter because of revenue that came in short of its expectations.

Spirit expects to report an adjusted loss of between $160 million and $173 million for the three months ended June 30, compared with a previous estimate for a loss of no more than $145 million. It expects sales of $1.28 billion, down from a forecast of at least $1.32 billion.

Spirit said non-ticket revenue, which accounts for the myriad fees long associated with its rock-bottom fares, came in "several dollars lower than anticipated" per passenger.

Shares of the budget airline were down about 6% in extended trading after the airline released its investor update in a securities filing.

The airline, along with rival Frontier Airlines, has recently revamped how it sells tickets by offering bundles that include things like seat assignments and carry-on bags that it used to sell a la carte. That brings its business practice more in line with larger competitors.

"As the Company progresses on its transformation strategy, it anticipates that over time it will be able to drive improvement in total revenue per passenger segment," Spirit said.

The company is facing several challenges, such as oversupplied U.S. domestic market, an engine recall from supplier Pratt & Whitney that has grounded dozens of aircraft and the fallout of a federal judge's ruling to block a planned acquisition by JetBlue Airways earlier this year.