The 95th percentile method for calculating a customer's billable transit volume has been the industry standard used by transit providers for over a decade due to its simplicity. We recently showed [1] that 95th percentile billing can be unfair, in that it does not reflect a customer's contribution to the provider's peak load. The 95th percentile method is also inflexible, as it does not allow a provider to offer incentives to customers that contribute minimally to the provider's peak load. In this paper we propose a new transit billing optimization framework that is fair, flexible and computationally inexpensive. Our approach is based on the Provision Ratio, a metric that estimates the contribution of a customer to the provider's peak traffic. The proposed mechanism has fairness properties similar to the optimal (in terms of fairness) Shapley value allocation, with a much smaller computational complexity.