To install the PRO Key you just go on Google PlayStore and buy it. Reboot your device after installation. Please do not remove free version of the application.

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To purchase the PRO Key you must associate a credit card or a prepaid card to your Google PlayStore account. In some countries you can also pay with phone credit and PayPal.

Payment methods accepted by the Play Store: full list

Yes, you just login to Google PlayStore with the same account that you used to purchase app and it will possible reinstall the applications that were previously purchased.
No, has not an expiration. It will be use for ever.
If your ProKey is invalid, please uninstall the free application and the ProKey. Reboot your device. Now go to PlayStore (with the same account used for purchase) and reinstall first the free application and then the ProKey purchased.
No, you can’t transfer the license.
The app is distributed by two different Stores: PlayStore for Android and iTunes for iOS.