Hi Anna,
Sorry for the delayed reply — the site was down when I tried to answer (which is a whole other issue). What you describe might be possible, I think, if you were to use a Google form for registration, although even then I’m not sure how you’d *require* payment.
We couldn’t do it with the built-in registration form that’s integrated with the THATCamp website you get when you register a THATCamp, though. That’s managed with an option called THATCamp Registrations. The problem is that it’s the same form that’s used for all THATCamps, so we haven’t been able to figure out a way to build in payment options that would be unique for each site.
Come to think of it, though, since we do have a few removable question fields, we might be able to add a removable plain-text field where you could in theory put a PayPal embed code for your site only. No promises, though: that might take developer time that we don’t have.
It sounds like you’ve already consulted the page, but just in case you haven’t, I’ll point you to the help page where the whole registration fee issue is discussed in more detail: http://thatcamp.org/help/organize/registrations/