protected function sumTotalWeight(&$total, array &$item, array $order)
    {
        $product = &$item['product'];

        if ($product['weight_unit'] !== $order['weight_unit']) {

            try {
                $product['weight'] = $this->convertor->convert($product['weight'], $product['weight_unit'], $order['weight_unit']);
            } catch (Exception $ex) {
                return null;
            }

            $product['weight_unit'] = $order['weight_unit'];
        }

        $total += (float) ($product['weight'] * $item['quantity']);
        return null;
    }