It can take time — and a lot of money — to save up enough airlines rewards miles to take the trip of your dreams. But imagine amassing the miles you need for a flight to Europe only to be hit with a $700 surcharge.

That’s apparently something happening more often to rewards users thanks to differences in carrier fuel surcharges — or carrier fees — for international flights, the Washington Post reports.

The additional fees often show up when passengers try booking a flight through a partner airline of the carrier they hold reward miles with.

For example, the Post reports that two flights from New York to London booked for travel between Oct. 5 and Oct. 12 through American Airlines and partner carrier British Airways had a fee difference of $558.

The flights, both booked through American Airlines’ website, were for the same route and the same amount of reward miles — 60,000.

Yet, the American flight has fees and taxes of only $171, while the British Airways flight had fees of $518 and taxes of $211, for a total charge of $729.

The Post points out that these charges are generally built into the price of a flight, but airlines aren’t required to do that with mileage redemption; reward miles only apply to the base fare for the flight.

A spokesperson for British Airways tells the Post that the charges appearing on the New York to London flight do not apply to flights within Europe.

“We always quote fares inclusive of all taxes, fees and charges, which apply to all our tickets, including redemption tickets,” she said.

As for the surcharges themselves, the Post reports that they can be for a number of things, such as fuel costs, but airlines aren’t required to disclose exactly what the fee funds are used for.

In the end, the Post reports the best way to avoid such high fees when using reward miles to book flights is to be aware of the routes and airlines they are using.

This fee could triple the cost of your reward flight [The Washington Post]

Want more consumer news? Visit our parent organization, Consumer Reports, for the latest on scams, recalls, and other consumer issues.