You can create custom taxes to add to an invoice by selecting the 'Manage Sales Tax' button at the top of the 'Invoices' screen. Note: Learn all about Invoices by clicking here.
From there, you will be brought to the Sales Taxes screen. On this screen, you can create, edit, or delete taxes.
If you choose to create a new sales tax, a pop-up screen will appear with all the options you need to create a new tax, which is as simple as a name and a tax percentage.
You can add these taxes to any invoice you create. Learn all about adding taxes to an invoice by clicking here.