Flag a customer contact as Accounts so they receive all Invoices
By default the primary contact will receive a copy of all automatic invoices and overdue notices.
You can setup a second "accounts" contact by creating an "Extra Field" called "Accounts" and typing a value into it (value can be anything, example: yes). Leave this "Accounts" extra field empty for other non-accounts users. Once this is done all automatic invoices and overdue notices will be sent to this email address, however other things such as Quotes and Job Emails will go to the primary contact like before.
You can change this "Accounts" extra field into a checkbox as well, this is a little nicer than typing "yes" into a text box. See Settings > Extra Fields