Discounts can be added to any invoice as a fixed amount or as a percentage of the total amount due.
Open the invoice.
Under the Payments section, next to the fee, click Mark as Paid.
In the Method dropdown, select Add a discount.
Enter a fixed amount or a percentage.
The invoice now shows the new total amount due, with the discount applied. You can now process the invoice payment.
To remove the discount from the invoice, click the trash icon beside the discount.