code: private function getItem($sku)
      {
          $className  = Config::get('shop.item');
          $item       = new $className();
          return $item->where('sku', $sku)
              ->where('cart_id', $this->attributes['id'])
              ->first();
      }
docs: Retrieves item from cart;
  
  @param string $sku SKU of item.
  
  @return mixed