I have a variable charging rate mainly to absorb any losses that may come from the job due the the client or the job being a pita. If they think I am too expensive then that is fine, if not then I have a contingency in the estimate. Where I don't need the contingency then the client gets a discount because the job was hitch free. I will happily tell the client that.
My fees have to cover my hourly rate and all my insurances and overheads for vehicles, workshop, machinery, etc as well as design and research time. I can sometimes spend several days of design time on a project that the client doesn't see and that needs to be paid for too.