Enpass costs one time fee per platform for lifetime license. Windows platform has 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 (excluding Desktops) running Windows phone 8 or Windows 8.1, you wouldn’t have to buy it for other.
When you purchase Enpass on Windows Store, it gets linked with the Microsoft ID from which you had installed and made payment to Windows Store. So to restore Enpass Pro version on any other Windows device, download the app from the same Microsoft account from which you had made purchase earlier and the Pro version would be automatically unlocked.
This process will going to restore your purchase only, not your data. So it is recommended that you take backups regularly OR enable sync with supported cloud.
NOTE: Full version of Enpass would be unlocked automatically after a couple of hours.