Microsoft recommends to organize multiple Azure subscriptions in the CSP partner system with the new Azure Plans. See a schematic representation of the management model here.
Here´s the documentation for Azure Plans. When an Azure subscription is transitioned in the Microsoft partner portal, with the "Transition billing to Azure plan"…
…a new level "Azure Plan" container is added between the customer and the Azure subscriptions. All CSP subscriptions will be added in that Azure plan. The organizational diagram looks as here.
Per customer one Azure plan can be created and the Azure Plan gets a new subscription ID (* the green boxes in the graphics, here ID 5 and ID 6). This is the "new" ID for getting the usage based costs. Customers get the benefit of seeing a basic cost estimation of their Azure costs (if configured) in the Azure portal in the cost management blade.
More to come about this topic in future…