WooCommerce add custom field to invoice

gistfile1.php

<?php
// add custom field to invoice email
add_action( 'woocommerce_email_after_order_table', 'woocommerce_custom_invoice_fields' );
function woocommerce_custom_invoice_fields( $order ) {
  ?>
  <p><strong><?php _e('Free Book:', 'woocommerce'); ?></strong> <?php echo get_post_meta( $order->id, 'Free Book', true ); ?></p>
  <p><strong><?php _e('Free DVD:', 'woocommerce'); ?></strong> <?php echo get_post_meta( $order->id, 'Free DVD', true ); ?></p>
  <p><strong><?php _e('Gift:', 'woocommerce'); ?></strong> <?php echo get_post_meta( $order->id, 'Gift Order', true ); ?></p>
  <p><strong><?php _e('Gift Message:', 'woocommerce'); ?></strong> <?php echo get_post_meta( $order->id, 'Special Gift Message', true ); ?></p>
  <?php
}
?>