Checkboxes next to each email address within a contact labeled "Auto Email". When checked, any time an invoice is posted for the contact it automatically emails that person a copy of the invoice.
This would be especially useful for AR accounts that do a high volume of work.
I know there is a checkbox in contact details to "Only Email Invoices" but that serves a different purpose and doesn't automatically email the invoice, and doesn't handle which email addresses to email it to.