Marking an item sold out
On the sell screen, press and hold the item tile and choose "Sold out for today". The tile fades and shows "Sold out"; tap it by mistake and the device says "Sold out – press and hold to bring it back."
It comes back by itself at 4:00 the next morning. Restocked sooner? Press and hold the tile and choose "Available again".
One option, not the whole item
Out of pearls but milk tea still sells: open the item’s options and press and hold that option. It fades with "Sold out"; the other options work as usual.
The other registers know too
Marking things sold out works offline. Other registers in the shop see it within about a minute. If the order you are ringing up has something that just sold out, the device says "Something in this order just sold out – check with the customer."