As long as the devices use the same Google Play accounts, you do not have to pay again to download iReal Pro to your other Android devices.
Open Google Play Store and tap on the 3 lines on the top left or your name on the top right and check if you’re logged in with the correct email address.
Google handles the billing and we have limited access to it - we can only look up purchases by email address or order number.
In case you don't remember what email you used for the original purchase, send us an email with all the possible email addresses and we'll look it up.
In order to move your purchase to another email address, you need to contact Google Support directly: https://support.google.com/googleplay/?hl=en#topic=3364260
The Google Play store and Amazon Appstore are separate entities, we cannot transfer your purchase from one to the other.