protected function getInvoiceLines()
    {
        $itemLines = $this->getItemLines();
        $itemLines = $this->addLineType($itemLines, static::LineType_OrderItem);

        $feeLines = $this->getFeeLines();

        $discountLines = $this->getDiscountLines();
        $discountLines = $this->addLineType($discountLines, static::LineType_Discount);

        $manualLines = $this->getManualLines();
        $manualLines = $this->addLineType($manualLines, static::LineType_Manual);

        $result = array_merge($itemLines, $feeLines, $discountLines, $manualLines);
        return $result;
    }