To obtain a 20% discount in the Perfectly Deep webstore, accessed at shop.perfectlydeep.com, one must become a premium Perfectly Deep member (Aficionado or Connoisseur.)
You can sign up to become an instant Aficionado or Connoisseur member on the front page of perfectlydeep.com. Scroll down to the headline 'Choose your membership,' and pick either of the two to activate your membership and webstore discounts.
Once you are a member, you will find the discount code in the Dashboard section of the Perfectly Deep website.