protected function validateItem($item)
    {
        // validate not null
        $this->validateNotNull('quantity', $item->getCalcQuantity());

        // validate discount
        $discountPercent = $item->getCalcDiscount();
        if ($discountPercent < 0 || $discountPercent > 100) {
            throw new PriceCalculationException('Discount must be within 0 and 100 percent');
        }
    }