doing it that way would cause no tax to be applied if the donation is in the cart and another taxable item is also in the cart, surely you would want to be able to tax the taxable item if they add both to the cart?
the way to handle taxes for specific items is to add the taxable column and mark the items as taxable or not.
that allows you to add both the taxable and non taxable item to the cart at the same time and apply the appropriate tax based on the subtotal of the taxable items.