/**
  * @link http://code.google.com/apis/analytics/docs/gaJS/gaJSApiEcommerce.html#_gat.GA_Tracker_._addItem
  * @param \UnitedPrototype\GoogleAnalytics\Item $item
  */
 public function addItem(Item $item)
 {
     // Associated items inherit the transaction's order ID
     $item->setOrderId($this->orderId);
     $sku = $item->getSku();
     $this->items[$sku] = $item;
 }