Tax percentages can automatically be added via the Settings view, under the `Tax Rate` section. If you're looking to add tax to a Products Order (items selected from the Products menu), please reference this guide: How can I include Sales Tax on my Product Orders?
After setting your tax rate, you can then update your amount and see the tax amount for that transaction.
And after the charge is complete, you'll see that your client was charged for the full amount (amount + tax).
In the event that you do not want to collect any sales tax, simply set the Tax Rate to 0%.