3D FlipBook - call the plugin using do_shortcode

I appreciate the plugin for its versatility and currently using it on our web-site.

We have many books for our students and would like the student to select the required book and view. In a Wordpress page, the shortcode works perfectly. But when called through a php, it won't work.

Code:

<?php 
require_once( 'wp-includes/shortcodes.php' );
require_once( 'wp-includes/plugin.php' );
    $selected = $_POST['Books'];
    if(isset($_POST['submit'])){  
    if(!empty($_POST['Books'])) {  
        $selected = $_POST['Books'];  
        echo 'You have selected: ' . "$selected";
        $flipbook_popup = '[3d-flip-book id="'.$selected.'" ][/3d-flip-book]';
        echo $flipbook_popup."
";
        //echo apply_filters( 'the_content',"[3d-flip-book id='$selected'][/3d-flip-book]");
        echo "do_shortcode($flipbook_popup)";
    } else {  
        echo 'Please select the value.';  
    }
    }
?>

The Result: You have selected: 16[3d-flip-book id="16" ][/3d-flip-book] do_shortcode([3d-flip-book id="16" ][/3d-flip-book])

Where I'm going wrong? Please help.