WooCommerce - Add extra columns to the shop order table

<?php
// WooCommerce - Add extra columns to the shop order table
// since v3.3

add_filter( 'manage_shop_order_posts_columns', 'set_shop_order_posts_columns', 99 );
function set_shop_order_posts_columns( $columns ) {
  // $columns['unwanted_column'] = 'Unwanted column'; // comment out any unwanted columns
  $columns['order_items'] = 'Order items';
  $columns['order_notes'] = 'Order notes';
  $columns['payment_method'] = 'Payment<br>Method';
  return $columns;
}

add_action( 'manage_shop_order_posts_custom_column' , 'show_custom_columns', 10, 2 );
function show_custom_columns( $column_name, $post_id ) {
  switch ( $column_name ) {
    case 'order_items':
      $order = new WC_Order( $post_id );
      $order_items = $order->get_items();
      $nr = 1;
      foreach( $order_items as $order_item ) {
        print $nr.') '.$order_item->get_name().'<br>';
        $nr++;
      }
      break;
    case 'order_notes':
      $order = new WC_Order( $post_id );
      $note = $order->get_customer_note();
      // choose one of the following print statements
      // print $note;
      print $note ? 'Yes' : '';
      break;
    case 'payment_method':
      $order = new WC_Order( $post_id );
      print $order->get_payment_method();
      break;
  }
}