title: Magento: Observer for order state is complete
body: <p>On Magento, I am developing a commission module which I need to pay my agents when the sales is completed and not when the order is placed.</p>
  
  <p>Ideally I would want to capture it when the order state turns into 'complete' but I have not found an observer to do this ?</p>
  
  <p>I could add this to the shipment observer but a order can have multiple shipment and hence not right.</p>
  
  <p>Alternatively I could always fire a cron job to calculate the order for last hour but again does not seem the right way.</p>
  
  <p>Any suggestions on what is the right way to do this.</p>