Once you buy koala (or any app or in-app purchase) for iOS or Android, you should have it for all your devices on that platform at no extra cost. However if you bought it on your apple device, you would need to buy it again on the google play store if you wanted to have it on android as well.
e.g. If you bought it from Apple, you don’t need to buy it again on any of your apple devices.
e.g. If you bought it from Google for your android device, you don’t need to buy it again on any of your other android devices.
e.g. If you bought it for your Apple device, you would need to buy it again for your google device.
This is the way it is – google and apple don’t have an agreement to share licenses between their platforms, I’m very sorry about that.