Once logged into your account:
Hover over the Account navigation tab and select the View Billing option from the drop down menu.
Click on the html link next to the listed statement number(s) to view your invoice or click on the pdf link to view and download your invoice.
You can view older invoices by clicking on the Show all link next to Monthly Statements.
If you have a current balance due, click on the Make a payment displayed next to your balance to do so.
Please note that domain registration is not a billed service, such as hosting. If you are looking to renew a domain name, hover over the Domains and select the Domain Renewal option.