The bottom of your Client Area dashboard displays a summary of due Zen Hosting invoices.
Alternatively, you can click Billing and then click My Invoices. From there, you can drill down into each individual invoice, by clicking the appropriate View Invoice button.
There are two other menu options you may have noticed when you clicked Billing.
The first is Add Funds. You can add funds and the money will be deducted from any future invoices that are generated.
The second is Mass Payment. Making a mass payment is a very easy way to pay the outstanding balances of your invoices in one payment.