En la tienda, Ud. paga (to pay) en ____.

“La caja” means “the checkout” or “cash register,” where you pay in a store. So the sentence is “En la tienda, Ud. paga en la caja” (“At the store, you pay at the checkout”). “El tamaño” is “the size,” “la goma de borrar” is “the eraser,” and “el empleado” is “the employee.”

AI-written and checked against the verified answer.