The Simple Taxes add-on is a quick and easy way to add sales tax to your customers' purchase.
To enable the Simple Taxes add-on in Exchange, go to the Add-ons page and click the Disabled button.
You'll then see a button with a gear symbol next to the Enabled button. Clicking the gear symbol will take you to your Simple Taxes settings.
There, you'll determine your default tax rate to charge and whether or not the tax is applied before or after any discounts are taken.