You can check your Google Play Store order number through the payment receipt sent to your Google account email or directly through the Google Payment Center.

[How to check order number in Google Payment Center]
 1. Access Google Payment Center (payments.google.com)
 2. Log in with your Google account
 3. Subscriptions and services > View purchase history > Select order and check Transaction ID on receipt
※ Please provide the order number starting with GPA or attach a screenshot