The charge is not for an extra seat per se. Say the helicopter, after subtracting fuel and the pilot, can carry five passengers or 1200 pounds. If the passengers are 260, 270, 140, 190, and 170, the total weight will be 1030, which is fine. However, if the passengers are 260, 270, 240, 245, and 230, the total weight is 1245, which would be over, so they may only be able to take four people even though two people paid for an extra seat. The charge is more to offset extra fuel costs for more weight carried. They never used to make you reserve an “extra seat” when you booked, but just weighed you before you got on and charged extra if you were over.