A store front stock shelf 300 installed together with a sales processing device 200 at a position of cash register detects a stock status of cigarette packs STP housed therein, and produces store front stock data SSD. A user operation terminal 100 installed together with the sales processing device 200 at the position of cash register acquires, from the store front stock shelf 300, store front stock data SSD of cigarette packs STP selected on the commodity selection screen TSG displayed in a switchable manner in response to enter operation by the ordinary user, and makes the touch panel display, in a switchable manner, a commodity selection screen TSG presenting “in stock” if the cigarette packs STP are in stock. In this way, a franchise add-on system capable of simplifying and smoothening purchase of even over-the-counter commodities not readily accessible by the ordinary users, such as cigarette packs STP, and capable of improving LTV and retention, can be provided.