i think
PHP Code:
$this->_responseText['priceTotal'] .= $currencies->display_price($this->_shoppingCart->total, $product_check->fields['products_tax_class_id']); 
should be
PHP Code:
$this->_responseText['priceTotal'] .= $currencies->display_price($this->_shoppingCart->total, 0); 
because, if $product_check->fields['products_tax_class_id'] > 0 , this tax-value is added to priceTotal; the tax was already added