For Joomla 1.5 do folowing steps:
Add new function to the end of 
components/com_jshopping/lib/functions.php. Here is the code of the function:
function changeCategoryTextUsePlugin($category){
    $mainframe =& JFactory::getApplication();
    $dispatcher =& JDispatcher::getInstance();
    JPluginHelper::importPlugin('content');
    $obj = new stdClass();
    $params = &$mainframe->getParams('com_content');
    $obj->product_id = $category->product_id;
    $obj->text = $category->description;
    $obj->title = $category->name;
    $results = $dispatcher->trigger('onPrepareContent', array (&$obj, &$params, 0));
    $category->description = $obj->text;
    return 1;
}Call ths function in 
/components/com_jshopping/controllers/category.php in
after
the line
$category->getDescription()
It must works.