I was using Kagi (the now-defunct store/payment API provider whose domain name later got bought by the search engine) at the time, and Kagi worked out at around that percentage for me* — though at the time, hosting and bandwidth was also a significant cost on top of that.

And this weekend, I'm starting to convert some of my old Mac (PPC era Java) shareware into web games so I can play them again. No ads, no cookies, hosted for free (for now) on github. I tried it out on my iPad, which didn't exist as a product when I wrote the original, and turns out this is fine.

* I can't be bothered to dig out the archive link for the pricing to figure out the exact percentage, there was both a fixed fee per transaction and a percentage fee that varied by payment method.