You Get the Silent Treatment
Nobody gets in a fast-food line to chit-chat with the cashier. You order what you want and go. That’s how it is, that’s how it will be, and nobody will ever change it. No, not even a Karen, who will usually throw a tantrum and call the police due to the slightest inconvenience.
Whenever a Karen places an order, the OP just silently records what they want. After the Karen moves away, the OP makes sure to be super-kind and helpful to the following customer. That’s what happens whenever you try to be extra to the person who doesn’t get paid enough to care about you.