Not all of us need to send off invoices as we are bricks & mortar companies - lots of us have sales that don't require this feature but also some sales that do.
Please add the option to "Save as Sent" to the invoice module !
Perhaps a hidden field could be added "send_later" which, when this option is selected we could use a checkBox on the invoice to:
Ticked = hidden field: send_later = true
unTicked = hidden field: send_later = false;
This way we can keep track of invoices that do need to be sent (perhaps via a filter, the api or deluge) !
& great for those of us that like to send invoices weekly, monthly etc.