If you had an Android phone, an iPad, a Windows PC and a MacBook you would need to purchase a licence for each as they are all different platforms, or more accurately different operating systems. If you have a Linux system then you are golden.
There is no web interface for Enpass, as your database is not stored on their servers, but on your choice of location.
Pricing is a one off cost, however if Enpass migrates from version 6 to 7 in the future then I would expect there would be a charge. This is unlikely to happen any time in the near future as version 6 has only been available for a few months.
Hopefully the information above is correct, apologies if I have miss something but I am doing my usual trick of answering forum questions late at night on three hours sleep.