Need to "think about it" before booking a flight so you can shop around? Did you make a mistake in your purchase and have to change it or cancel? 

According to Department of Transportation regulations, if you’ve booked a flight in the U.S. and you’ve bought a non-refundable ticket seven days ahead of the flight, you're entitled to hold your reservation and the fare and to change or cancel your reservation within 24 hours without paying a cancellation fee.  Typically those fees can be up to $200 on the large carriers for a domestic fare, but they can go as high as $450 for some international fares.  See this chart for more information.

You can cancel the reservation or change it within the 24-hour window. If you change it, a fare difference may apply, but there is no penalty. This applies not just to U.S.-based airlines, but to any airline selling flights in the U.S.You still have to pay for the airfare, and then get a refund without penalty, unless you’re booking with American Airlines, which lets you to hold your seat and the fare for 24 hours without paying for it. On American, you should not pay for the fare, but merely choose the 24-hour hold option without payment. American also sells fare "add-ons," starting at $68 round-trip, that allow you to change your flight for free at any time. Add-ons include a checked bag round-trip and priority boarding, so that’s something to consider.

Southwest Airlines lets you change or cancel a fare without penalty within the 24-hour window, and it also allows you to change or cancel a reservation anytime before the flight and get a credit for the full amount of your fare, applicable to future travel within a year of the original reservation. You will have to pay any applicable fare increase, however.

Allegiant Airlines is a bit more specific, stating in its rules that you may cancel as long as your scheduled flight is at least 168 hours (one week) away at the time of booking.

More On This...

In order to take advantage of the 24-hour cancel or change rule, it's best to book directly with the airlines, either online or by phone, rather than through third-party websites.

And it goes without saying that you can cancel a fully refundable ticket anytime and get a refund, though there may be a fare difference if you change rather than cancel.

Frequent Flyer Award Tickets, Too?

Does this apply to frequent flyer tickets? I've been able to cancel frequent flyer reservations and get all fees refunded and miles re-instated without penalty, most recently on British Airways. But the DOT rules are unclear on this, and US Airways clearly states that the 24-hour cancellation rule does not apply to frequent flyer tickets.

Rule 260

One more thing: many people don't realize that in airline contracts there's a rule (often called Rule 260) about "involuntary refunds." Basically, it states that if the airline refuses to carry you for any reason, or if your flight is delayed more than a specified amount of time (more than two hours on American Airlines, for example), or if the flight is canceled, you can apply for a full refund, even on a non-refundable ticket. Here, for example, is Hawaiian Airlines' Rule 260. United calls its rule on this something else, which you can see by wading through its contract of carriage.

So let's say you buy a fare you no longer can use and the DOT 24-hour rule doesn't apply. Your only hope to avoid the change/cancel fee is if your flight is canceled or severely delayed. It may or may not be worth your time to show up for your flight and pray it's canceled or significantly delayed (you do have to check in for the flight). You can also get a refund if there's a significant schedule change before your departure (let's say they change you from a 9 a.m. departure to a 6 a.m., or your new flight requires a much longer layover or an overnight stay, or you’re changed from a nonstop to a connecting flight). Here, for example, are the rules on this from American Airlines (this information is provided for travel agents, but applies no matter how the fare is booked).

George Hobica is a syndicated travel journalist and founder of the low-airfare listing site