In some countries, tax is levied on most goods and services provided by registered businesses. If this tax is relevant to your online business, you can specify the tax rate(s) in your E-Commerce settings. To do so, go to the Cogwheel Icon > E-Commerce > Settings > Tax.
To add the tax, first select a country from the drop-down list, Country, enter the Tax Rate and then click on Add.
For US you can specify the tax rate for entire country or individually for each state.