Product modification items are not sold separately, they function as add-ons for the main products and meals you sell. However, you can still apply different prices and taxes to them. Here's how to set taxes.
1. Navigate to the Catalog on the left sidebar.
2. Expand the Product Modification Items menu and select the store where your item is located.
3. Using the dropdown menu select tax/taxes. A success message will appear notifying that the item has been changed.
There you are! Here's how the item looks with taxes set. Learn more about how to create taxes.