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).
Printed copy · Anthology Volume 3, page 1345Printed copy from The Tender For Law Anthology, Volume 3, page 1345 (printed 13 June 2014). Added by this site; not part of the Facebook export.