Bad news mate, when I take the rear wheel off the Busa which has a 300x18 on it i tell them straight no marks or it's 1500 for a knew one do you want the job, luckily for me and them never a mark on it.
The problem you had was the guy doing it didn't give a @*!#, I also use a place up here in the Midlands that a proper cheapo tyre place but they looked after my rear 20s as if they were made from glass, next time it doesn't matter were you take them insist on watching, if you don't like anything tell them,