I think it is a high quote from that shop. Are they going to allign your car after installation? If so, how long will the warranty be on the allignment? I doubt that they would. I would shop around somewhere else.
In all honesty, you can do it on your own because since it is a kit, it is easy as removing the old one and putting the new one in..
I was the one who put lowering kit of my car without having any experience on it, to my experience it was not hard at all. But, if I were you I would do the install myself that way I will learn something about my car.
As always remember safety is first. If you are uncomfortable then I would not do it.