When you log in to the billing portal, you will see all new unpaid bills listed under the Open tab. After making a full payment against a bill, you can move it to the Closed tab. To avoid the hassle of manually closing bills, you can enable the “auto-close” option in the Settings tab, which automatically marks bills as closed once they are paid in full.

Billing Portal Open Invoice to Closed Invoice
Billing Portal Settings Open Invoice to Closed