How to add new custom submenu under another plugins menu

There is plugin called Shopp in my WP admin page , this plugin has got top level menu “Shopp” .
This is the top level menu:

$menus['main'] = add_menu_page('Liquor Shop', 'Liquor Shop', SHOPP_USERLEVEL, 'shopp-orders', array(&$this,'orders'));

And I’ve created some plugin which need to add as submenu under “Shopp” top level menu , so it is adding sub menu ( link …..wp-admin/admin.php?page=ach-faq.php ) but when I am clicking on submenu it shows “You do not have sufficient permissions to access this page.”

Debug result:

Pagenow = admin.php
Parent = shopp-orders
Hookname = shopp_page_ach-faq
Menu = Array
Submenu = Array
Menu nopriv = Array
Submenu nopriv =
Plugin page = ach-faq.php
Registered pages =

My code:

function ach_faq_menu(){
	add_submenu_page('shopp-orders', 'My FAQ Plugin', 'My FAQ Plugin', 8, __FILE__, 'section_1');
}
function section_1(){
	echo 'Text';
}
function section_2(){
	echo 'Text';
}
add_action('admin_menu', 'ach_faq_menu');

How can I fix this ? Please help me !