In my case I finally figured out why I have credits for customers who I know never ended up paying us in the first place! When I would be crediting an invoice because I knew the customer was never going to pay us the rest (after paying the diagnostic fee) I would credit the total amount not realizing they'd already paid the prior fee on that invoice (for the diag portion) this would ultimately leave a negative balance as a credit for that name! Ugh...but that's a real thing...for us.