permalink and $_GET

Is the reason you are adding the post_id to the URL because you are you trying to access the post_id variable in the template? You can do so much more easily by accessing it in the Loop of your template file

<?php 
if ( have_posts() ) : 
    while ( have_posts() ) : 
        the_post();
        // Two ways to get the Post ID //
        echo $post -> ID; 
        echo get_the_ID();
    endwhile; ?>
endif; 
?>