I understand that you think it's overpriced for something simple and it is when compared to doing it yourself on this specific car, but that seems to be a flat-rate price and not all cars are that simple to change a bulb. Some can get pretty involved. My local inspection station would charge slightly more for the same service so it's probably not as an excessive charge as you'd think.
And you get to save some time and effort with driving over to her, or her to you, buying bulbs and doing it yourself. In the meantime, your daughter is driving a slightly safer car with all 3 middle brakelight bulbs on instead of just 1, so that's a win.