Passengers can cancel a Delta Airlines flight within 24 hours of purchase, according to the airline's 24-hour cancellation policy.
https://www.airtripsadvisor.com/delta-airlines-cancellation-refund-policy/
https://www.airtripsadvisor.com/american-airlines-cancellation-refund-policy/
https://www.airtripsadvisor.com/united-airlines-cancellation-refund-policy/