public static function checkComboSetItems($mes, $cart) {
        // just quit if none of the cart items are combo-sets
        $found = false;
        foreach ($cart as $crow) {
            $sels = unserialize($crow['serial']);
            if (!empty($sels['comboSetId']) && !empty($sels['comboSetItems'])) {
                $found = true;
                break;
            }
        }

        if ($found === false) {
            return $mes;
        }

        $divcheckmes = self::checkDivisions($cart);
        if (!empty($divcheckmes)) {
            return $divcheckmes;
        }

        return self::checkCartItemsInventory($mes, $cart);
    }