in the webassist/plugins/shopping_cart/cart_editable.php and cart_readonly.php the code that is used to display the tootle looks like this:
<tr class="eC_SummaryFooter">
<th class="total">Order Total:</th>
<?php
$total = $WA_Store_Cart->GrandTotal()-$WA_Store_Cart->GetShipping();
if($total < 0) $total = 0;
?>
<td class="total"><?php echo WA_eCart_DisplayMoney($WA_Store_Cart, $total); ?></td>
</tr>
Change that to this to add a new row to show the GST amount:
<tr class="eC_SummaryFooter">
<th class="total">GST:</th>
<?php
$total = $WA_Store_Cart->GrandTotal()-$WA_Store_Cart->GetShipping();
if($total < 0) $total = 0;
?>
<td class="total"><?php echo WA_eCart_DisplayMoney($WA_Store_Cart, ($total / 11)); ?></td>
</tr>
<tr class="eC_SummaryFooter">
<th class="total">Order Total:</th>
<td class="total"><?php echo WA_eCart_DisplayMoney($WA_Store_Cart, $total); ?></td>
</tr>