on the set_up_cart.php page, try changing the code order for the session and the get info call from:
<?php
// Set session variables
$_SESSION['TaxRate'] = 0.19;
echo 'Session TaxRate is '. $_SESSION['TaxRate'];
?>
<?php
$dmbc_in_stock->GetContent();
?>
to having the getInfo() functor first:
<?php
$dmbc_in_stock->GetContent();
?>
<?php
// Set session variables
$_SESSION['TaxRate'] = 0.19;
echo 'Session TaxRate is '. $_SESSION['TaxRate'];
?>