You can set a different tax class for your add-on field than for the main product by using the ‘Products’ field type (available in the Pro version of Add-Ons Ultimate).
- Create a new product for your add-on option. If you would like to hide this product from the main shop pages, see this article
- Assign a tax class to this product
- In your main product, add a ‘Product’ field then add your new add-on product(s) as the child products
- Assign a tax class to your main product
Now, when users select one or more options from your main product, their prices will be assigned a different tax class from the main product.