Yes! You are able to edit a pending invoice by adding a custom line item for one-time charges, discounts, or other incidentals. You can add a line item to a pending invoice the same way you would a due invoice. You can also remove and void items from pending invoices.
Access the pending invoice from the Invoices > Pending Invoices page of your web dashboard
Click into the invoice you'd like to edit
Select 'Add item' to create a one-time charge or discount
Need more info on Invoices?