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(+2)

So you really should call the demo a demo. Especially if you're never going to do public release for any more of the game.

Are you charging for each new version or is it a one time buy?

On this site, the payment system works like this.  You paid for the purchase of a certain bar.  new updated versions for this bar will always be available to you.

If you buy version 3.0, when 4.0 comes out you will have 3.5 available. 4.0 will be available to us in place of 3.5

See that's how a lot of them do it but not all so I feel it's best to simply ask rather than assume. Some creators will charge individually for each version released.