A **walk-up** is an apartment in a building that lacks an elevator.

A **walk-in** is a person who comes into an establishment without an appointment or without having phoned beforehand.  A walk-in order is an order placed by such a person. Many different kinds of establishments refer to "walk-ins" to describe some of their customers: health clinics, car dealerships, restaurants, spas and salons, and so forth.



You can *walk up* to the counter and place an order. But such an order would be called a *counter order*, not a walk-up order.