We have a trade counter, and as such create invoices through one customer account called "cash".  However, a lot of customers like an email copy of their invoice, like the service offered by Argos at their counter.  Therefore the ability to enter the persons email address direct into the invoice as each one will have a different email address, click an email button and it will send to that entered on the invoice and save at the same time.  Currently the only way is to go into the cash customer account and change the email address, save it then highlight the invoice and send from email that way, but as mentioned if we have several customers in all being served at the same time there would be no way to do this.