[Observium] Billing

I have the subscription version and have the billing working. What I am trying to do is work out if I can only bill certain traffic. i.e. I only want to charge for international traffic at a $ per MB rate but charge nothing for local traffic. I'm using a Mikrotik Cloudcore router, the international data goes out one gateway and the domestic out another. So I really only want to bill the data for a port that goes in and out one particular gateway.

Anyone doing anything like this?

Thanks,

Mike.