As account owner (the one who created the account), you may download your invoices at any time. Just go to the Account settings in the top right corner and then go to Invoices.
New invoices get created at the end of each month. The account owner will receive a short info within his CentralPlanner. We might also send new invoices via email to any of your email addresses. In order to setup this email address, go to Billing address & Payment data just next to the Invoices link. At the end of the form, you may add the Email address for Email delivery.