Invoices are automatically sent to the email address associated with your account on the first of every month. Due to the cost (both monetary and environmental) we do not typically send out invoices in the mail, however you can request that paper invoices can be sent to you for an additional $3 monthly service fee.
We are working on an update that will allow you to view any open invoices and/or balance through a secure web portal, however, so keep an eye out!