You can tweak your paysite to run so smooth that you can more or less avoid support issus. Having said that you can find your self kneehigh in capital letters emails from more or less enraged members at any time (usually AOL noobs).
So get ready for 24 hour a day support jour, or rent support but that as so much else will drive up your overheads.
Most third party billing companies have a 48 hour support clause. Ie your bound to give support response within 48 hours or risk loosing their service.
A paysite is like a greedy baby screaming for breastfeed around the clock. And still there might be little of value pooping out in the other end
I have found promotion to be the hardest and most time consuming part. Making a kick ass site is relatively easy.