I emailed them, they said that once a recurring billing starts, I can't change the amount billed. So if someone upgrades to a higher plan, I'd have to cancel the current billing and have them fill out the form again for the new recurring payment.. plus I can't force charges for overages. They have a nice site and pretty cheap prices, but for a hosting site, they just aren't any good.
netbilling -- thanks, I'll check it out.