public function getInvoiceNum()
    {
        $sQ = 'select max(oxorder.oxinvoicenr) from oxorder where oxorder.oxshopid = "' . \OxidEsales\Eshop\Core\Registry::getConfig()->getShopId() . '" ';

        return (( int ) \OxidEsales\Eshop\Core\DatabaseProvider::getDb()->getOne($sQ, false) + 1);
    }