Invoices can be reviewed on a per-domain basis. To review a domain's invoices:
From the Custom Billing interface, select the domain for which you would like to view invoices.
Click "invoices".
In the Invoices interface, you will be presented with a list of invoices for the domain you have selected.
Click on an invoice to view it. You can manually set an invoice as "paid" by placing a checkmark beside "mark as paid" and clicking "apply" or "ok".
You can also email a copy of an invoice to a customer by clicking "email invoice".
Note: if the domain you selected has not completed at least one billing cycle, you will not see any invoices.