When commodities are packaged on the premises of a store, shop, or other establishment selling foodstuffs and when such commodities are offered for sale by weight, a scale or other weighing device that shows the weight shall be placed within plain view and conveniently near the display, counter, or case containing said commodities, which scale or weighing device shall be for use by prospective purchasers of the commodities. Such scale or weighing device shall be inspected and tested by the Director or his designee as frequently as he determines is necessary.