Login to the WebControlCenter
at https://www.webcontrolcenter.com/customer.aspx using your Customer ID and password.
For login help, please see, KB 1088
From the Menu options, go to Customer > Billing > Billing Admin.
On the Billing Admin page, you will be able to see the balance for your site(s) or server(s).
If you would like to pay your balance via a credit card or check, please see KB 968