>is there a shortcode to delete (empty) the order?
no.
actually i don’t even understand how what you are describing above works
of course i dont know the ins and outs of your setup so am guessing a bit, but if the waiter is using the system and the customer is using the system at the same time the waiter cannot empty the customers order/cart as the sessions will be completely independent/different from each other (unless they are literally using the same tablet )