Android: Google Play.
The purchase is done according standard Google Play store terms through the Google Play store client application. The license is valid for the unlimited number of downloads, installations and updates for the lifetime of the e-mail account. The e-mail account must be registered in the Accounts section of the settings of Android device and Google Play Store client application. The validation of the license is done through the standard means of Google Play store application/service installed on the Android device.
Android: Directly from the developer.
This purchase does not provide registration in the Google Play store. It is direct purchase from the developer. The application package (APK) must be downloaded, installed and updated manually from the web site. The application package (APK) is always available for a download from the Download section of this web site. The e-mail account which is used for the purchase must be valid and registered in the Accounts section of the settings of Android device as IMAP/POP3 e-mail account. The license terms: unlimited downloads, installations and updates for the lifetime of the e-mail account. Refunds are not possible.
Android: Samsung Apps.
Neutron is available in the Samsung Apps but please note that such purchase will be valid only for Samsung devices with Samsung Apps application installed. Thus the best universal alternatives for Android platform are Google Play or PayPro.
Apple: App Store.
The purchase is done according standard Apple App Store terms through the App Store client application. The license terms: unlimited downloads and installations for the lifetime of the Apple ID.
BlackBerry: BlackBerry World.
The purchase is done according standard BlackBerry World terms through the BlackBerry World client application. Supported platforms are Playbook and BlackBerry 10. The license terms: unlimited downloads and installations for the lifetime of the BlackBerry ID.
General: Purchase transfer from one platform to another.
It is not possible to transfer purchase from one platform to another, app stores do not provide such possibility or interface to the app developers.