Taxes for options. Is it possible?

I have products which are downloadable, but they have an option, which is a DVD so it needs shipping.

If someone buys the product without the option DVD, I don’t want taxes to be applied, but when the option is set, I DO need the tax to be applied (it must be applied depending on product cost, not on shipping cost).

I don’t find the way to do this. If I set the tax for the product, it is always applied. I only need it when the option is bought with the product.

Hope you can help. Thanks.