Replace WooCommerce Subject line Preview Text ( drop in /wp-content/mu-plugins/ folder.

klbs_custom_woo_pre_header_text.php

<?php
/**
 * Plugin Name: Replace Pre-Header Email Text.
 * Description: Replace Line 27 with what text you want. This will replace the text just after your Subject - "Your text".
 * Author:      Keith Crain ( Kronoslabs.io )
 * License:     GNU General Public License v3 or later
 * License URI: http://www.gnu.org/licenses/gpl-3.0.html
 */

/**
 * Pass custom email header to woocommerce.
 *
 * @param string 'woocommerce_email_header' Hook name to hook function into
 * @param string 'klbs_custom_email_header' Function name to be hooked
 */
add_action( 'woocommerce_email_header', 'klbs_custom_email_header', 10, 2 );

/**
 * Add preview text after subject line.
 */
function klbs_custom_email_header( $email_heading, $email ) { 
	echo '
          <div style="max-height: 0px; overflow: hidden;">
             Insert hidden preheader text here.
          </div>

          <!-- Insert &#847;&zwnj;&nbsp; hack after hidden preview text -->
          <div style="max-height: 0px; overflow: hidden;">
            &#847;&zwnj;&nbsp;&#847;&zwnj;&nbsp;&#847;&zwnj;&nbsp;&#847;&zwnj;&nbsp;&#847;&zwnj;&nbsp;&#847;&zwnj;&nbsp;&#847;&zwnj;&nbsp;&#847;&zwnj;&nbsp;&#847;&zwnj;&nbsp;&#847;&zwnj;&nbsp;&#847;&zwnj;&nbsp;&#847;&zwnj;&nbsp;&#847;&zwnj;&nbsp;
          </div>
      ';
}