One way you could do this would be to add 19% to the price of the item when it is being inserted by the add to cart server behavior. In your add to cart code find the place where the price is being set like this:
$ATC_itemPrice = floatval("<your price>");// column binding
Just after this add in this line:
$ATC_itemPrice += ($ATC_itemPrice * .19);// column binding
This will ensure that the price of the item in the cart will include the tax in it. Next you can add in a row to the cart display that will show you how much tax was applied to the order. To do this go to your cart display page and look for the "Total:" line. You should add in a new row just above this one that looks like this:
<td><?php echo WA_eCart_DisplayMoney($eCart1, ($eCart1->GrandTotal() - ($eCart1->GrandTotal() / 1.19))); ?></td>
This will ensure that the users can easily see the amount of tax that has been applied to the order. You will need to update this code to use your cart object instead of the default $eCart1.