Negative balance

Hello, can I make a negative balance?

I want to make a dropshipping site. For example, if the buyer does not pay for delivery, there will be a balance of -60. And the client will know how much he owes.

And I wanted to add my own tab and add some functions there but I need to edit the plugin. And after updating the tab will disappear. It will be very cool if I can add my tab and do something there ( Add HTML, widgets, text or finished page )