public function getGroupItemsCart($postdata) {
        global $usces;

        // There is no way to pass parameters to `->inCart()` so we have to overwrite $_SESSION['usces_cart']...
        $currentcart = $_SESSION['usces_cart'];
        unset($_SESSION['usces_cart']);
        add_filter('usces_filter_realprice', [$this, 'filterGroupItemRealPrice'], 10, 2);
        add_filter('usces_filter_in_serialize', [$this, 'filterGroupItemInSerialize'], 10);
        foreach ($postdata as $post) {
            // overwrite $_POST since `->inCart()` takes no parameters...
            $_POST = array_merge($_POST, $post);
            $usces->cart->inCart();
        }
        remove_filter('usces_filter_realprice', [$this, 'filterGroupItemRealPrice']);
        remove_filter('usces_filter_in_serialize', [$this, 'filterGroupItemInSerialize']);
        // get the group items cart we just created
        $rawcart = $_SESSION['usces_cart'];
        $builtcart = $usces->cart->get_cart();
        // restore the actual cart to its original state
        $_SESSION['usces_cart'] = $currentcart;

        return [
            'rawCart' => $rawcart,
            'builtCart' => $builtcart,
        ];
    }