In my POV, you can't accurately charge for Twilio usage for a particular zone. What if a customer sign up in one zone stays there for some time then moves to another zone. It's practically not possible to charge a zone for the SMS usages, You have to consider it as the business expense and charge commission from zones accordingly.
Previously I had made some reports like number signups in each zone for a particular period of time based on the signup locations - this is not to charge the zones but for analytical purposes. And increase commission for a zone if there are lots of orders & signups coming from that particular zone.