This hack could save you hundreds of dollars.

It can be frustrating to book a flight and then later see the same ticket on sale for significantly less. With most airlines, there’s nothing you can do about a fare drop except pout. If you’re flying Southwest Airlines, however, you’re in luck.

Southwest Airlines has a fare drop program: If you purchase a ticket and the price goes down, you can rebook (for free) at the lower rate and get back the difference in price.

All you have to do is log on to Southwest’s website and rebook your reservation at the lower fare via the “Change/Cancel” option that will be displayed. You can also call Southwest directly to rebook at the lower rate.

The type of refund you’ll receive depends on which fare you purchased.

For a refundable fare, you can choose to receive the difference in fare as a refund to your original method of payment or as a flight credit.

If you bought a nonrefundable fare, the difference in price will be returned via a flight credit for future travel. If the ticket is a Wanna Get Away Plus fare, the flight credit can be transferred to another Rapid Rewards member.

For any ticket that was purchased using points, the points associated with the difference in fare will be returned to your account. 

How to Take Be Alerted to Price Drops? Unfortunately, you can’t set up an automated alert on Google Flights or similar flight search sites to notify you if your itinerary becomes cheaper—Southwest does not allow third-party sites to display its fares.

Instead, set yourself a reminder to frequently check the price of your flight to see if it’s dropped. And subscribe to Southwest’s email newsletter (or follow it on social media) to learn about sales that may indicate your trip cost has gone down.

Twist’s Take: If you’re flying Southwest, don’t forget to keep tabs on your flight and apply for a refund if its price drops.