In HikaShop, the end step of the checkout, like all the steps, uses the same URL index.php?option=com_hikashop&ctrl=checkout&task=step&Itemid=183
Now, if you want, you can go to Display->Views and edit the file end of the checkout view and add a redirection like that :
$app =& JFactory::getApplication();
$app->redirect( JRoute::_('index.php?option=com_content&view=article&id=***',false) );
Where *** is the id of an article where you can thank your user, etc.
Then, you can use the URL index.php?option=com_content&view=article&id=*** for the tracking.