The Department of Transportation has a rule that could save you money on your next flight.

If you find a lower fare within 24 hours of booking a flight to or from the US, and your flight is a least seven days away, you can get a full refund of your original purchase price and buy a new one at the lower rate.

Some airlines and third-party sites give you a better deal and allow you to cancel the tickets for free, even if your flight is less than a week away.

If it’s been more than 24 hours since you booked, you could still save as some airlines will still honor the cheaper rate but the refund will be in the form of travel credit.