protected function verifyUnitPriceField($value)
    {
        $this->verifyRequiredField('unit_price', $value);

        if (in_array($this->item_type, [self::ITEM_TYPE_DISCOUNT, self::ITEM_TYPE_STORE_CREDIT])) {
            $this->verifyNegativeField('unit_price', $value);
            return;
        }
        
        $this->verifyNonNegativeField('unit_price', $value);
    }