excluding add-ons from sales tax

I’m managing an ecommerce store that sells clothing items, some of which can be monogrammed for a fee. Right now sales tax is calculated on the total cost, including monogramming charges, but technically the monogramming charge should not be taxed. How do I adjust the tax calculation so it doesn’t include an add-on charge like monogramming, but just the base price of the product? The monogramming is handled with the Gravity Forms plugin.

Thanks!