It sounds like you might need to make the additional comics as separate projects if you want people to have to pay again? Not sure, I haven't really messed around with separate prices for individual files.
Nah, about that one I asked support and I certainly can set separate zips with different prices! Now I just want to know about the editing files part. x'D
If no one knows around I'll have to send an email I suppose, but had to try (!)
