To be completely honest. I don't know. On my side, it's labeled as "early access". So you should pay once and own it forever with every updates that will come in the future. But I didn't try it myself so I wouldn't bet my life on that...
Hi, I bought Summer Scent one year ago to download the latest Version at that time and was able to download the 0.6.1b version in November. You only need to buy it once.
