title: How can I validate purchases with Amazon IAP API
body: <p>In an IOS app, I normally validate that purchases have succeeded and been processed by my application by comparing Apple's list of receipts with one that I add to locally when I've processed the purchase myself.  This safeguards against things like app crashes during purchase, bugs, etc that can make things get out of sync.  I'm trying to do something similar with Amazon.
  I have come across statements like this repeatedly in Amazon docs about purchase validation/restoration:</p>
  
  <p><code>It is possible for a purchase transaction to have more than one Purchase Token representation, so they should not be used as transaction IDs.</code></p>
  
  <p>This was a huge help as I thought the exact opposite.  Unfortunately, I haven't found the rest of the thought: "The transaction ID can be found here:...".  I'm starting to think that their transactions don't offer a stable ID...!  Please tell me what I'm missing or, if it really doesn't exist, how I can safeguard against the problems I mentioned above?  </p>