There is a more elegante way to display the correct Grand Total.
In the Bindings Panel in the eCart clumn there is a record "PreTaxSubTotal".
That value holds the correct amount.
Best is to insert that code becide the php-code of the "Grand Total" of the page -
and then copy&paste the part you need to bring the "PreTaxSubTotal"-value in the "old Grand Total"-code. Then delete the "PreTaxSubTotal" field - you just brought in.
The new code looks (in my case) like this:
<?php echo WA_eCart_DisplayMoney($eCart_en, $eCart_en->GetTaxableTotal()); ?>
It displays now the correct value.