The US plans to fine American Airlines $90,000 for failing to disclose fees related to people voluntarily giving up their seats for a free flight, the Department of Transportation said Monday.

Passengers who received flight vouchers would have to pay $30 ticketing fees in the transaction, the agency said.

Carriers can offer any type of compensation agreed to by a volunteer giving up his seat, but are required to pay passengers cash compensation in cases where the customer is involuntarily "bumped" from the flight.