Make it compatible with Taxonomy Tax Module

http://drupal.org/project/uc_taxonomy_tax

In our case, we have a XML loaded from feeds with the products info and one piece of data is the tax value, in our case (4, 8 and 18). We are using uc_taxonomy_tax so each product have its own tax value. We are not using classes but because of that Ubercart 2 VAT dont work as expected because we are not checking the "product" class option in the tax settings, only setting the taxonomy as the taxonomy tax module request to avoid double tax.