We have upgraded our Exchange server(s) to Exchange 2013. By default all existing accounts are still running live on the previous Exchange Server and we've been upgrading each subscription to the new system.
During this process we have to switch the Subscription Plan so the provisioning system associates the old subscription with the new subscription running on the new Exchange Server. This initiates an Order with a $0.00 balance which you will receive via email. No action is required on your end. Simply disregard.