If you are using an iOS device, you can find the receipt by logging into your iTunes account and selecting "Purchase History".
If the purchase were made more than 90 days ago, please follow the steps provided in the following link: https://support.apple.com/en-us/HT204088 to locate the receipt.
Google Play:
If you are using an Android device, you can find the receipt by searching for “Google checkout” in the email you use with Google Play.
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