I think the major point of this article is for engineers to focus on innovation for business, rather than optimizing infra to support app new trial. Then a scalable and agile stack is critical for choice.
A relational database scalable for the billing data volume, should be a best answer, in term of realtime process and analysis，say complex query.
Suggest an article of a big logistics company case here, not only billing, but also order, messaging, transshipment:
https://en.pingcap.com/blog/how-to-reduce-cost-per-order-wit...