Accessing Order-id after order created.

I have an SAP integration where the contents of an order are passed to an SAP server by a System Plugin when the order is saved. To complicate matters the Joomla site is accessed from the customers purchasing system which uses the same login account for all orders and multiple employees can be creating orders at the same time. This is OK because all orders are kept separate by the session id.

Here is my problem. Currently the system plugin gets the order details by calling up the last order id for the customer which works 99% of the time but occasionally returns the wrong details when two order are created simultaneously. When I use $this->order nothing is returned as it seems the variable has already been removed. What is the best way to keep the order_id as a global or session variable so it is available after the order is created? ie. What file would I declare this in and in what format?

Found it. The code below seems to do the job.

$app =& JFactory::getApplication();
$order_id = $app->getUserState('com_hikashop.order_id');