U.S. airlines now get one-fourth of their revenue from sources other than fares -- mostly fees -- and one of the biggest is a charge of up to US$200 to change or cancel a ticket.

The fee galls consumers who find themselves with an unexpected need to change their travel plans. In some cases, the fee is more than the price they paid for the ticket.

When all the receipts are counted, it is likely that consumers paid the airlines more than US$3 billion in fees to cancel or change a flight in 2015. That is triple what they paid in change fees in 2007.

Avoiding these hefty fees will take a bit of planning before you book your flight. Once you pay for the ticket, you're at the mercy of the airline. Experts have some tips:

Grace Period

If your flight is at least seven days away, federal regulations require airlines to give passengers 24 hours to change their mind at no cost.

Airlines can let passengers hold a reservation at the quoted price for 24 hours before paying, as American Airlines does, or let them cancel without penalty for 24 hours, as most other carriers do.

Exception Airlines

Southwest Airlines doesn't charge change fees. If you cancel at least 10 minutes before the flight you can use the ticket's value for another flight, but you could owe more money if the new flight has a higher fare.

Alaska Airlines lets passengers change or cancel for no fee if the flight is at least 60 days away.

Ticket Types

Refundable tickets cost more -- sometimes substantially more -- but they might pay off if you have any doubts about your ability to make the trip.

Also, some airlines offer bundled fares that include a free ticket change. "The Works" from Frontier Airlines costs more than a basic ticket, but includes a free change. You also get free bags and priority boarding. And it is refundable.

"Refundability is better than (a free) change because you get to say, 'I don't want to go.'" says George Hobica, founder of airfarewatchdog.com.

Some experienced fliers suggest that you buy two one-way tickets if you can replace either one for less than the change fee on a round-trip ticket.

Here's why:

Most airlines now sell one-way tickets for half the price of a round trip. Adam Goldstein, the co-founder and CEO of Hipmunk, a travel site-comparison firm, says he recently did this for a San Francisco-Los Angeles round trip. When he needed to change the first leg, he threw away his US$80 ticket but was able to buy another for about the same price instead of paying the higher change fee.

Time Your Purchase

If you book your flight many months in advance there is a greater chance that a crisis at work or an illness could cause you to miss the trip. Goldstein says you should book ahead of time because fares do rise closer to the flight, "but wait until you are as confident as possible that you're actually going to make the trip."

Wing It

Finally, "You hope and pray your flight is severely delayed or canceled," Hobica says, because you are entitled to a refund if the airline can't honor your ticket. He says you should demand a refund even if the flight's schedule has changed significantly - if the departure time changes by several hours or a nonstop becomes a one-stop.

There are also outfits that will help travelers avoid hefty change fees by locking in a fare for several days without buying the ticket. It is like buying an option - you can walk away if your plans change, and it protects you from a sharp rise in the fare.

A start-up called Flyr lets you lock in a fare for up to seven days for a fee of US$5 to US$35 depending on the flight without buying the ticket, says the firm's product manager, Andrew Jing. There are limits to its service - American Airlines doesn't allow its fares to be displayed on Flyr or a similar service offered on Hipmunk. Some airlines including United offer their own versions of locking in a quoted fare for a fee.

U.S. airlines raised US$2.98 billion from change fees in 2014, the latest full year for which government figures are available. Through the first nine months of 2015, revenue from the fee was up more than 2 percent, putting the airlines on a path toward raising about US$3.05 billion from the charge in all of 2015. That's up from US$915 million in 2007, and as extra charges on airline travelers go it is second only to baggage fees.