You don't buy them per se, we just give early access to our Patron supporters. If you are not subscribed the month a new update comes out, you won't be able to play it.
That being said, we are currently working on a huge overhaul update, transferring our game from being a parody into an original project, and after that, as soon as we are sure all of the major flaws have been ironed out, we will probably publish our game on Steam for everyone's convenience. Then it will be a one-time purchase.