WooCommerce - Add content to email headers

<?php
// WooCommerce - Add content to email headers
// Email templates are not affected by this method
add_action( 'woocommerce_email_header', 'custom_email_header', 20, 2 );
function custom_email_header( $email_heading, $email ) {
  $order = $email->object;
  print 'Order No. '.$order->get_order_number( ); // example
}