After purchasing an item on the app store, you will receive a confirmation email with a receipt. If you can't find your confirmation email, you can view your purchase history online. To view your history, please follow the steps below:
Apple Store:
1. On your iOS device go to Settings -> [Your name] -> iTunes & App Store
2. Tap your Apple ID -> View Apple ID -> Purchase History
Google Play Store:
1. Go to payments.google.com and log in to your Google account
2. Select "Activities" to view your order history