Enpass has been developed and released as a native app for each platform where the desktop version is FREE and the pro version for mobile devices is chargeable. Since you have to purchase through the App stores available on your device, your purchases are handled by the respective App stores only. Such a license is non-transferable between the platforms and you have to buy it separately for each app store.
On Windows platform, we have linked the purchase with Microsoft's associated purchase system, that means Enpass app for Windows phone and Windows 8.1 would be considered as one at the time of purchase. Once you buy Enpass from same Microsoft account on any of your devices running Windows phone 8 or Windows 8.1, you wouldn’t have to buy it for other.