I found this Part of the Code in the express.php:
// Remove shipping tax from total that was added again in ot_shipping
if (DISPLAY_PRICE_WITH_TAX == 'true') $order->info['shipping_cost'] = $order->info['shipping_cost'] / (1.0 + ($quotes_array[$default_shipping]['tax'] / 100));
$module = substr($shipping['id'], 0, strpos($shipping['id'], '_'));
$order->info['tax'] -= tep_calculate_tax($order->info['shipping_cost'], $quotes_array[$default_shipping]['tax']);
$order->info['tax_groups'][tep_get_tax_description($module->tax_class, $order->delivery['country']['id'], $order->delivery['zone_id'])] -= tep_calculate_tax($order->info['shipping_cost'], $quotes_array[$default_shipping]['tax']);
$order->info['total'] -= tep_calculate_tax($order->info['shipping_cost'], $quotes_array[$default_shipping]['tax']);
Obviously this Code subtracts the tax from the shipping cost. But if I change anything on this Code the calculation on the first call to PayPal (after the checkout_payment.php) changes. But as I said that one was correct. So I am still stumped why the calculation after the checkout_confirmation.php is different from the one after the checkout_payment.php. How can I change anything, that has only affect on the final call to PayPal?