The invention relates to a method and system (100) for selling an additive (101) for a product (102). Additives are consumer goods which are used in conjunction with another, usually more durable good. The system (100) is notified when a user (130) orders a product (102) for which an additive (101) is available. It then estimates a point in time at which the user's inventory level will be too low, and presents a notification (114) to the user (130). This notification (114) includes an ordering mechanism (116) to order new stock. This system obviates the need for inventory tracking mechanisms and so especially useful for additives.