I wouldn't be paying for the fanfics,
though, I'd be paying for the handy convenient tool that helped me download and format them. Like how I paid money for an ebook reader that I mainly use to read fanfics. The stories themselves are still free, the ebook reader just makes it more convenient to read them.
Fortunately, it seems I've used your epubmerge plugin a couple of times before. And I recall it being very helpful, certainly worth donating to support. So there, everything's all nice and morally pure.