The function addEcommerceItem() has no return value.

Due to your documentation this function should be called before e.g. doTrackEcommerceOrder() is called.
If you do so you will get

Fatal error: Call to a member function doTrackEcommerceOrder() on null

In our opinion you should implement the fluent interface and return the object instance:

689 = $this->ecommerceItems[] = array($sku, $name, $category, $price, $quantity);
690 + return $this;

Thanks in advance for feedback!
Have a great time!