The Spanish board for approving transportation, decided last week that the carriers are not allowed to charge for excess luggage, as long as it stays within the allowed weight limit.

Will probably be handled by the carriers in some smart way, but this is the present situation. Don't know if it applies only to El Hierro, but should probably cover the jurisdiction area of the Ministerio de fomento in Spain.

http://www.canarias7.es/articulo.cfm?id=199983