Not to say that the camera shop didn't get the dust on the focusing screen, but dust is a common problem. You will probably get dust on it again even without having anything done to the camera. Some older film cameras had focusing screens that were easily removed, but even cleaning those was sometimes a pain because every time I cleaned the screen and replaced it, I'd find a new speck of dust that got on the screen during the process. A little dust on the screen is annoying, but it doesn't harm anything.

Alan