I’m building a service billed for via usage. As others have pointed out, I have to solve metering for a variety of reasons (beyond billing) and won’t take an external dependency for it. It’s a nonstarter, metering is just too central.

Also, any invoice system that’s based on query-time aggregation is probably too expensive and definitely a risk. Fees are accumulated in real time, incrementally - not when I generate an invoice.

But probably the biggest miss I encounter is around auditing. Customers dispute bills, and always will to some degree. The process of resolving those disputes needs to be automatic, clear and understandable, and based on something that the customer can cross check (ideally).