title: WooCommerce: Add custom meta as hidden order item meta for internal use
body: <p>I want to add some meta data to the order item in WooCommerce.
  These meta fields are for internal use only and shouldn't be visible.</p>
  
  <p>We have some extra fields in the product like an extra fee. I want to use that fee later to work with after I export the orders. </p>
  
  <p>I found a very good answer here: <a href="https://stackoverflow.com/a/41988701/1788961">https://stackoverflow.com/a/41988701/1788961</a> </p>
  
  <pre><code>add_action('woocommerce_checkout_create_order_line_item', 'add_custom_hiden_order_item_meta_data', 20, 4 );
  function add_custom_hiden_order_item_meta_data( $item, $cart_item_key, $values, $order ) {
  
      // Set user meta custom field as order item meta
      if( $meta_value = get_user_meta( $order-&gt;get_user_id(), 'billing_enumber', true ) )
          $item-&gt;update_meta_data( 'pa_billing-e-number', $meta_value );
  }
  </code></pre>
  
  <p>But with this example, the content from the meta fields will appear in the order details for the customer.</p>
  
  <p>Is there a way to make these fields only visible in the backend and usable for internal functions?</p>