At the moment we can only discount invoices, not add extras. There are lots of perfectly legitimate reasons why a seller might need to add an extra to an invoice. In my own case, for example, I offer an authoring service on About Me templates. I'm sure lots of people offer embellishments on crafted goods, such as personalisation or extra trims. The only way to achieve this at present is by negative discount - naturally this looks pretty odd and can confuse/worry customers.
Please add an "extras" box alongside the "discount" one. Or make the existing box work as an adjustment, plus or minus - though this would involve reversing the way the box works at present and might confuse some sellers. I don't mind which way it works, I'd just like the facility without frightening my customers. Thanks.