Q: I recently purchased tickets for travel to San Jose, Costa Rica, from San Francisco. Our daughter is just 8 weeks old and will be flying in her mothers’ lap, not in a seat. How is it that her ticket was almost $500? I asked the lady at the counter if she made a mistake, she said she has no control over fares posted on her computer. She said normally the airfare for a lap child is approximately 10 percent of the applicable adult fare. How did it get to be almost $500?

A: It depends somewhat on when you bought the fare for your lap child. If you show up at the airport last minute without notifying the airline that you have a lap child, they might be charging 10 percent of a very expensive last minute "walk up" fare rather than the cheaper fare you might have bought far in advance. Also, some airlines add the fuel surcharge and taxes onto any lap child fare. The general rule is that airlines charge this 10 percent lap child fare only on international flights. Even if you’re traveling on a frequent flyer ticket, the airline will charge you 10 percent of what the adult ticket would have cost had you paid for it. Currently, no U.S.-based airline charges for lap children on domestic flights.