public static function inCartCheckComboSet($postdata) {
        global $usces;

        /**
         * Check stock and item options for the combo-set item itself
         *
         * Note that at this point we haven't checked options or stock for group items so there may be a case
         * where the following `$usces->incart_check()` exits before we can construct group item error messages.
         *
         * However, by default, Welcart will use JavaScript to check that all required options are selected
         * for the item. Therefore, `$usces->incart_check()` will only exit with an error if there
         * is no stock. If there is no stock then it doesn't matter whether group items are valid
         * since the item can't be purchased anyways which is why we make the call here.
         */
        $usces->incart_check(); // will exit to the item page if any checks fail

        // back up the $_POST object
        $backuppost = $_POST;
        foreach ($postdata as $post) {
            // overwrite $_POST since `$usces->incart_check()` takes no parameters...
            $_POST = array_merge($_POST, $post);
            // Check stock, options, etc for each item.
            $usces->incart_check(); // exits to the cart page if any checks fail
        }
        // restore the $_POST object
        $_POST = $backuppost;
    }