You bought the Pro version from an iPhone or an iPad
You will get a receipt by Apple or check your Apple id purchase history. Just follow the instructions on this page. https://support.apple.com/en-us/HT204088
You bought the Pro version from an Android device
You can find your receipts on this link: https://support.google.com/store/answer/6160235?hl=en
You bought the Pro version from the web
To find your receipt, you can:
Make sure your are logged into your PhotoRoom account
Tap on the person icon at the top right of the screen
Tap on "Manage subscription"
You can view your receipt here