I really don't mind charging for software, as developers need to eat as much as the next person. $39 for something like this is really not excessive.

But I think it could have been made clearer that (1) you only download one file but it loads more from their CDN, it's not truly "single file" and (2) the free version is heavily limited and gated with a pop-up.

That said, I like the demo - it looks really slick and has a lot of features.