I'd be weary about using these on a regular basis. The car has to be clean and so do the letters otherwise you could get some really ugly swirls on the clear coat. As long as you put that in your legal that you aren't responsible for the damage you should be good. Also provide detail explanation that car surface needs to be spotless and dry before sticking on.
I'd still be terrified that kids would start sliding them around my car like a toy. Or even worse... spelling something terrible and me not realizing it!
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