5%的超售，是想take advantage of the cancelled bookings or no shows, becasue cancellation is typically covered by travel insurance and no show basically forfeits the points used for booking.
When that does not happen for a given flight, UAL should not pass this to the passengers.
另外， voluntary 只是乘下一班吧？ 那 Voucher 是不值钱的， 只叫 cash value, 又不能 Trade 。。。
