Hello,
From version 3.0 ViArt offers you the possibility to set a tax value per product type. This value will overwrite the base tax. For this go to Admin > Orders > Tax Rates > Edit Tax and specify the tax for each product type.
There are four default product types in the ViArt Shopping cart: Product, Accessory, Gift Voucher and Bundle.
You may also need to set a tax value to your own product type. For this you have to add a new product type going to: Admin > Products > Products Types > New Product Type. When your product type is added you can set an appropriate tax value in the Tax settings.
Please, read more information on tax rates setting at:
http://www.viart.com/setting_up_shopping_cart_tax_rates.html
With regards,
ViArt Support Team