in the Power Store side menu, the top level categories will not collapse and expand. Only a sub category that has further sub categories will be able to collapse and expand.
so if you where to create your categories as:
Parts
+Exhaust
--Aprilia
--BMW
--Cagiva
--etc....
+Mufflers
--Aprilia
--BMW
--Cagiva
--etc....
you would be able to get the effect you are after. you will also need to edit the webassist/plugins/side_menu/nested_accordion/sidemenu.php page, find the following line:
<script type="text/javascript" src="functions.js"></script>
and change it to:
<script type="text/javascript" src="../accordion/js/accordion.js"></script>
<script type="text/javascript" src="functions.js"></script>