I was referring to how to buy it, in the tab that appears when I click "Download", I put whatever price it is (in this case 9.99 or more) and I would already have access to the premium version and future updates, right?
Yes you permanently keep your products. You will be allowed to update your game as long as the dev allows it. Most devs do allow it but some of them dont. Just check the update logs and its fairly easy to tell.
