After creating an additional tax (which you can find out how to do here) you can add it to each individual product by:
1. Go to "Innoventory"
2. Click on "Products"
3. Click on the "Edit" button on the product you would like to add the tax to
4. Scroll down to "Pricing" and click on the space "Add-On Tax"
5. Scroll until you find the tax you created (note: do not add general or state tax to this space)