To easily find your Transaction ID for you Google Play Android Account, follow these steps:
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Go to your Google Payments page to the Subscriptions and services > Other purchase activity, you can click here: https://payments.google.com/payments/u/0/home#oneTimePurchase
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Find the transaction, normally the oldest, with the description "Android"
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Click it and a popup on your right will show up, with the transaction information, but also with the Transaction ID.