/**
 * Add custom content to a specific customer order email
 */

add_action('woocommerce_email_before_order_table', 'cc_add_custom_content_specific_email', 20, 4);

function cc_add_custom_content_specific_email($order, $sent_to_admin, $plain_text, $email) {

  if ($email->id == 'customer_completed_order') {
    echo '<h2 class="email-extra-title">TAX INVOICE</h2><p class="email-extra-text">Smith & Smith Pty Ltd<br />
    26 Smith Street, Smithville, NSW 2000<br />
    Tel: +61 (0)2 9876 5432<br />
    ABN 11 111 111 111</p>';
  }

}