Q. On our return trip from Chicago my wife and I gave up our seats because the flight was oversold and we each received a $400 voucher for future travel. Can these vouchers be redeemed only at the United desk at an airport or can we do it online or on the phone? Do all major airlines have the same policies?

A. Not surprisingly perhaps, some airlines don't make it easy to use vouchers. United and American, for example, don't allow you to enter the voucher number on their websites and book travel that way. You either have to do it at an airport ticket counter (or city ticket office, but those are far and few between; AA only has three left). You can also buy your ticket on the phone and tell the agent that you have a voucher; you'll be asked to mail the voucher and the value of the voucher will be deducted from your fare once it's received. I recommend that you make a copy of the voucher. Delta, in contrast, allows you to apply the voucher's value online.

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