In conversation with a manager some time ago, and I asked them what happens when they run out of items, they told me that they rely on good old fashioned guesswork for their beer orders, and they said that the tills don't have a part in the ordering process. In other words, if they run out of something, then it's down to not enough being ordered, or a big run on the item they've run out of, e.g. more cider sold in summer than in winter.