app\models\Invoice::markInvitationSent PHP Method

    public function markInvitationSent($invitation, $messageId = false, $notify = true)
    {
        if (!$this->isSent()) {
            $this->invoice_status_id = INVOICE_STATUS_SENT;
            $this->save();
        }
        $invitation->markSent($messageId);
        // if the user marks it as sent rather than acually sending it
        // then we won't track it in the activity log
        if (!$notify) {
            return;
        }
        if ($this->isType(INVOICE_TYPE_QUOTE)) {
            event(new QuoteInvitationWasEmailed($invitation));
        } else {
            event(new InvoiceInvitationWasEmailed($invitation));
        }
    }