Stuart Stone
Ask the question:
Who is the 'You' you are referring to?
Notice of Mistake, then return their offer VOID, No Contract.
If you want to be a bit feisty, attach your fee schedule informing that any further work required to again address their mistake will attract the following fees & may be construed as intimidation, coercion, harassment (or whichever of these terms is listed in your local legislation as an offence).