Create “new pages” within the admin section of plugin

The easiest way is to add another $_GET parameter to the URL. In the moment you are viewing admin.php?page=Customer_Search. You could make your links go like admin.php?page=Customer_Search&type=your-page.

In your plugins files you can then output the desired content based on that parameter.

if($_GET['type'] == 'your-page') {
   //output your page ...
}elseif( ....other pages ...