On Monday, the Department of Transportation (DOT) fined a half dozen airlines a total of more than $7 million for failing to provide timely refunds to customers. The department’s intervention assisted the airlines in issuing over $600 million in refunds.

Transportation Secretary, Pete Buttigieg says, “As people get ready to fly this holiday season, I want customers to know D.O.T has their back.”

Airlines have a booming business this summer, as more people have shifted their expenses from goods to transport and other services, respectively.

The Transportation Department says, “The refunds are meant to compensate passengers for flights that were canceled, significantly delayed, or otherwise substantially altered.”

Airlines based in Denver; Frontier Airlines was fined $2.2 million more than any other company. As per the announcement on Monday, it was the only US airline penalized with almost $222 million in refunds.