Vendor invoice Discount & Extra charge
To enter a discount and/or an extra charge for the invoice, press "Actions" and then "Discount & Extra charge":
You may add them whether as a percentage or flat amount.
To do that, select the corresponding extra charge and discount type (Percentage or Absolute) and enter the requred value:
Press "Save" after entering, and you'll go back to the Vendor invoice page.
Note: Invoices with zero or negative amounts cannot be linked to payments.