Exclude add-on charges from the discount calculation

I've set up a tiered discount campaign ('Logobolde tiers') where customers get a % discount based on quantity purchased. The product (e.g. Wilson Triad - Logobold) has customization add-ons like "Tryk på begge sider" (+30 kr per dozen), which are managed through an options/customizer app. The issue: Discounty is applying the discount to both the base product price AND the add-on price. The discount should only apply to the base product — not the add-on/customization surcharge.