After you make a purchase on Google Play, a confirmation email with your order information is sent to the Google account you used.

The order number will begin with "GPA" followed by a series of numbers, such as GPA.1234-1234-1234-12345.

If you can't find the receipt in your inbox, follow these steps:
1. Go to this website
2. Find Other purchase activity.
3. Select View Purchases.
4. Select an order to see your receipt.

Here's an example: