If you have standard tax enabled, it will function together with Avatax and you might need to disable standard taxes. Check System -> Localization -> <a target="_blank" href="#admin#rt=localisation/tax_class">Tax Class</a><br>Also don't forget to enable extension <a href="#admin#rt=total/avatax_integration_total">Avatax Integration Total</a><br>Important! Avatax Integration Total calculation order in extension totals must be after subtotal. Subtotal calculation should precede this extension total calculation. Check this in extensions total settings.