Please make sure that you've already create a tax under "Tax Settings"
Go To Payment Tab
Navigate to the "Payments" tab.
Select the Invoice
Select the invoice you want to add a tax to.
If you need to create a new invoice, you may want to check out our "How To Create A New Invoice" guide.
Add A Tax
Click "Add Tax"
Select the Tax
Check the box of the tax you'd like to add.
Save The Changes
Click "Save"
Save the Invoice
Click "Save" in the upper right corner.
