IIRC there's basically one payment processor, whose name I can't recall, that ~100% of NSFW sites use. And their fees are very high, given the risk of processing those payments.
CCBill's fees I think are like 11-15% plus $1k per year, pretty insane but underscores the risk
I could be wrong, but I didn't find a clear breakdown of who qualifies for each plan on their website. I'm guessing the 3.9% plan and their non-profit plan are there so they can present as a normal payment processor.