Adding credits will not affect an outstanding bill, and the bill cannot be paid by credits.
The outstanding invoices screen should give you an option to pay each monthly billing that might occur. Such usage can occur by running out of credits and continuing while it takes minutes or hours to actually be shut off will be put on a monthly bill. It also might be from before everyone was switched to pre-pay billing in February through April.
Check here for bills: https://platform.openai.com/settings/organization/billing/history
If you have left your bills unpaid for a year they will be gone from that screen and then you will have a REAL hard time getting them paid off and your account restored, despite having fed it new money.