The Department of Transportation has handed out fines to Allegiant and Hawaiian Airlines for breaking consumer-protection rules.
The carriers were hit with fines of $250,000 and $125,000 respectively.
The DOT said it found Allegiant failed to refund payments made within the required seven days on numerous occasions.
It was for refunds where customers unwittingly bought tickets for infants under two years.
The DOT also ruled against Hawaiian Airlines for violating regulations over handling and the reporting of involuntarily bumped passengers.
The airline says it has since revamped its internal reporting procedures in this matter.
Hawaiian also failed to correctly report disability-related complaints.
