Insert Shortcode exactly at the end of the content

It looks like you are using the Fanciest Author Box plugin ( FAB ).

This plugin is not open source so it’s therefore difficult to give a detailed solution, since I can’t look at the code inside it.

I assume it uses the the_content filter with some priority p.

To move the FAB box below the Sharethis & Related-Posts, you can then try this:

/**
 * Append sharethis & related-posts to the content
 * See http://wordpress.stackexchange.com/q/116264/
 *
 * @param string $content
 * @return string $content
 */
function the_content_wpse_116264( $content ){

    if( is_single() ){
        $content .= do_shortcode( '[sharethis]' ); 

        if( function_exists( 'wp_get_related_posts' ) )
            $content .= wp_get_related_posts();

    }

    return $content;
}

add_filter( 'the_content', 'the_content_wpse_116264', 11, 1 );

where the priority 11 is hopefully lower than p.

If this still gives you problem regarding the priority, you could try to remove the the_content filter applied by FAB and add it again with the relevant priority.