When generating or editing an unpaid or draft invoice, you can apply a certain discount of either a flat dollar or a percentage on the total due amount. Consequently, the invoice tax applies to the post-discounted total amount.
Adding discount post-tax calculation
To add a discount to an invoice after calculating tax:
Tap Invoices in the side-bar menu
Hover on a saved invoice and tap Edit
Navigate further down and input a figure in the Invoice discount field