Wow, that's a tough one. Just spitballing, I'd say:
1). Don't try to build it yourself, no matter how simple your case. Your needs will ALWAYS evolve to be more complex AND keeping up with the thousands (or more) of individual regulatory regimes in the world is a massive task.
2). At any given point in time, you need to know the state of everything required to rate, bill, and invoice at any other point in time. (i.e. bitemporal). As a consequence, critical data effectively becomes immutable, auditing is possible, and allows pre/post views of things after corrections.
3). Once you start to get into tiered/volume discount, combo discounts, providing multiple services, etc. complexity rapidly increases, so observability into how exactly things are calculated is of paramount importance. This one is probably pretty obvious, but missed a lot. How many times have you received a bill with little to now information about how things were calculated? It's possibly a business decision, but just as likely the process is a black box and not readily observable.
