WC_Email_Customer_Invoice::trigger PHP Method

        public function trigger($order)
        {
            if (!is_object($order)) {
                $order = wc_get_order(absint($order));
            }
            if ($order) {
                $this->object = $order;
                $this->recipient = $this->object->get_billing_email();
                $this->find['order-date'] = '{order_date}';
                $this->find['order-number'] = '{order_number}';
                $this->replace['order-date'] = date_i18n(wc_date_format(), $this->object->get_date_created());
                $this->replace['order-number'] = $this->object->get_order_number();
            }
            if (!$this->get_recipient()) {
                return;
            }
            $this->send($this->get_recipient(), $this->get_subject(), $this->get_content(), $this->get_headers(), $this->get_attachments());
        }