From https://eventespresso.com/topic/promotion-in-checkout-process-viewing-order/

move_promotions_row.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

// move promotions table row to just before total table row
// 
function move_promotion_table_row() {
    ?>
    <script>
      jQuery( document ).on('ajaxComplete ready', function() {
        jQuery('table#spco-payment-info-table .spco-grand-total')
          .before( jQuery('table#spco-payment-info-table tr.item:last') );
        });
    </script>
    <?php

}
add_action( 'AHEE__payment_options__reg_step_start', 'move_promotion_table_row' );