wp custom field for a page id

//add in function.php

<?php 
add_action( 'add_meta_boxes', 'flatfee_field' ); 
	function flatfee_field() { 
		global $post;
		if($post->ID == '40')//40 is page id
		add_meta_box("flatfee", "Flatfee", "add_flatfee", "page", "normal", "high");
	} 
	function add_flatfee(){   

		  global $post;
		  $custom = get_post_custom($post->ID);
		  
		  $flatfee = $custom["flatfee"][0];
		  ?>
		  <input class="custom_input" name="flatfee" value="<?php echo $flatfee; ?>" />
		  
		  <?php
	}

function update_flatfee( $post_id ) {

	if($post_id == 40)
	{
		update_post_meta(40,'flatfee',$_POST['flatfee']);	
	}

}
add_action( 'save_post', 'update_flatfee' );

?>