Great coverage on those answers. Thanks!

One followup: have you considered handling the actual usage calculation and aggregation? One use case that comes to mind is accepting API request logs from (CloudFlare|CloudFront|Logstash), processing them, and directly deriving billing from those. That moves the entire process outside of a system your customer has to touch (in cases where complex application-layer knowledge isn't needed). One less thing for a potential customer to worry about (and removes one of the reasons to need read access to a database in many cases).

Again, all the best! Happy hacking. :)