ScottyP

I also own a brake shop and in my first year I did install customer supplied parts on occasion. Over time I noticed a pattern with these people, they rarely showed up! Most likely they found some backyard guy to do it for $20 and a 12 pack, but didn't have the decency to call the shop to cancel. I quit doing it, no exceptions, and it doesn't bother me one bit if they don't like our policy. Let them go, they are not going to hurt you and only cause you grief. Focus on doing top notch work with quality parts and charging a fair price and you'll do just fine. After 4 years now I'm the go to brake guy in town and can barely keep up with the work.
 
Oh and if they need a reason I tell them it's a liability issue with our insurance company, that if their parts fail the insurance company has no recourse. They usually understand that. I even get one now and then that'll bring his parts back and let me do my job.

Wish I had some clear cut answers for you since no shows are a killer, can't imagine 60% stiffing you and I do consider a no show stiffing you cause that's lost time and income. During out first conversation and booking the appointment I usually get a feel whether they'll fall into that group or not, and if my gut alerts me I tell them to make sure they give me 24 hours notice if they can't make it, and I drive that point home pretty firmly. The tell tale signs are almost always money related, if they flinch at the price, if they ask if they can buy the parts (#1 big red flag), etc but almost always price related. It's also usually younger customers, specifically males in their 20's with older cars. I think they have buyers remorse over making the appointment or found Joe Blow shadetree to do the work for less.
 
If they stiff me once the only way I'll put them back on the schedule is if they leave the car and keys with me. Once it's in my possession I'll get it on the schedule and they're going to have to wait for an opening, no way they're going to get me twice. Good luck.