Most of our games that include in-app purchases have a "Restore Purchases" button which asks the app store that you purchased the game from to re-send which purchases you have made. You will usually find this within the Store screen (signified by a shopping cart).

For this to work you will need to be signed into the same account as you used when you originally made the purchase.

Unfortunately it isn't possible to transfer purchases between different operating systems (from Apple to Amazon to Google to Nook etc.) as each one uses their own purchasing system for their own devices.