public function getTotalDiscountSum()
    {
        $dPrice = 0;
        // subtracting total discount
        if ($oPrice = $this->getTotalDiscount()) {
            $dPrice += $oPrice->getPrice();
        }

        if ($oVoucherPrice = $this->getVoucherDiscount()) {
            $dPrice += $oVoucherPrice->getPrice();
        }

        return $dPrice;
    }