Add a message to the beginning of a WooCommerce order email. Change which order email by updating what is currently set to 'customer_processing_order'

woo-email

<?php 

add_action('woocommerce_email_before_order_table', 'custom_order_header', 20, 4);
 
function custom_order_header( $order, $sent_to_admin, $plain_text, $email ) {
    if ($email->id == 'customer_processing_order') {
        echo '<p>Once your order is processed...</p>';
    }
} 

?>