Right. It just needs to be a generic payment system. Much as we had premium rate dial up ISPs for a while to access porn (Apparently).
It could just be a very simple setup:
1. webapp notices you're on ISP A
2. webapp asks ISP A to bill you $3/month
3. you get an email from your ISP asking for authorization
4. you click the link and it's added to your bill
But this falls flat once you remove the assumption that a user only accesses the internet from a single ISP. So what happens when I decide to make that purchase on my lunch break at work? Or from the free wifi at the coffee shop?
There was a time that this would have worked, but I think there are too many people that aren't tied to a single physical location for their internet services. And we haven't all switched to something provided by a cell company.
It could just be a very simple setup: