Validate Promo Code and Deduct Price

I am trying to offer a $100 discount to the general public when booking a tour but ONLY if they choose a specific Price List Option.

I have a [promocodevariable] Custom Field set to Hidden and Default Value of: WINE100

I have a [promocode] Custom Field set to Text that is only displayed if a specific Price List Option is chosen.

Is there a way to to use Custom Rules, Fee Formulas, or Validation Rules that will deduct $100 if the correct Promo Code is used by the general public? 
For example: validatepromocode]equals[promocodevariable used in attached image.
 
Thanks,
Brian