It depends on the app market used for payment.
APP 📱
1. Google Play Store
Google Play Store > Profile (top right) > Payments & subscriptions > Budget & order history > Select the transaction > Capture screen including Transaction ID
2. App store
App Store > Profile (top right) > Purchase History > Select the order > Capture screen including Document Number and Order ID
WEB 💻
A receipt will be sent to the email registered under your Mnet Plus ID.
※ If you have trouble finding your receipt, please contact your payment provider.