Note: you must have the 'Account Administrator' privilege to view billing information ('Company Administrator' will not suffice).
Your invoices and billing history can be found by clicking 'Account' from the left-hand menu. To the right of 'Company Details' you'll see the Billing Panel, which includes Active Employees (the number you'll be billed for for the current month), Payment Method and Billing Address. Clicking 'View Billing History' will bring up a list of all your past invoices. Click any of them to view the details of that invoice.