If you cannot locate your purchase receipt, here’s an extended tutorial on where to find it, depending on the merchant used for your purchase.
 

Google Play (Android users, purchasing from a phone or Tablet)

 
Mobile Device:
  • On your mobile device, open the Google Play Store app. Tap Menu Account to locate your `Purchase History` section.
PC:
  • Visit http://wallet.google.com  and log in with the same account used to make your purchase with. All purchases should be listed under Transactions. Select the purchase which wasn’t received. 
     
    OR 
     
  • Visit http://payments.google.com  and log in with the same account used to make your purchase with. On the left, click ‘Subscriptions and services’ and then ’View purchase’. Select an order to see your receipt.
 
You may visit Google Play Support for more information: https://support.google.com/googleplay/answer/2850369?hl=en
 
An example of receipt: