$this->arrForm["order_id"] = $objQuery->nextval("dtb_order_order_id");
        ↓
    $this->arrForm["order_id"] = $this->getRandomOrderId();

    function getRandomOrderId()
    {
        $min = 1;
        $max = 2147483647;
        $objQuery = new SC_Query();
        while (1) {
            $order_id = mt_rand($min, $max);
            if ($objQuery->count("dtb_order", "order_id=$order_id")) {
                continue;
            } else {
                break;
            }
        }
        return $order_id;
    }