vbnet

Agreeing with several others: I think the airlines should take care of it programatically during the reservation. If any of the passengers on the reservation are below a designated age, the fee for choosing a seat should automatically be added and the purchase should HAVE to select seats before the reservation is complete.

Now, how to handle it through Expedia will be more complicated but if it cannot be done while the reservation is being made, perhaps a program can be in place to evaluate those types of reservations after they have been made. Send an email requiring confirmation that they need to pay $ to book seats together. It's not really an issue of forcing them to pay for an option to book "a seat" but the option of booking "multiple seats together."