title: Token that identify the user
body: <p>I'm developing an android application and I would like some functionality of the application is not free.</p>
  
  <p>I have thought to use in-app Billing Version 3 API, so I have defined an "In-App Product" in the developer console.</p>
  
  <p>After reading the documentation, I know that when I start the purchase flow I should pass in a string token that helps the application to uniquely identify the user who made the purchase.</p>
  
  <p>But how could I obtain a string token that identify the user?</p>
  
  <p>Thanks</p>