Have you got Insurance? Might be a good idea to think about it. If for some reason their machine manages to crap itself when you are working on it, it WILL be your fault (As far as the customer is concerned). As for killing a Customers machine, I have never managed it in the obscene number of years I have done this as a profession/on the side, but I am still insured.
Not sure who would provide that service in the US (There will be a company somewhere, they will insure you for anything, if you can afford to pay), but it might be worth looking into, if only to have another bragging right on your advertising.
One other thing. You do NOT charge for labour. You charge for Expertise. One looks like anyone can do it and is over inflated, the other reflects a skilled profession charging the going rate. It's just semantics, but it works, and is true for the most part.I might have already said it, but it works for me.
For example. Plumber goes out to a faulty valve, hit's it with a hammer, and hands over a bill for 300. When queried how he can charge 300 for 10 seconds work, he says this "I havent charged you for hitting the part, but i have charged you 150 for knowing what part to hit and 150 for knowing how hard to hit it"
Yeah, it's Old, Cheesy, but also very very true in our profession.