Re: ... Flat rate
No. No the ISP does not. My ISP charges me for the bandwidth. I get some in my monthly bill, and the rest I pay as I go.
Even "flat rate" is pay per GB in disguise. It's the ISP who decide to offer fate rate, so their decision if they can or cannot deliver such a service. They must advertise/provide a different service that is all they can provide/sell.