It looks like one core issue was designing the system around pre-determined bounds (monthly, yearly, etc). This happens in lots of systems and isn't specific to billing. A job scheduling system will have the same problem (we built it to track seconds and now they want us to track calendar dates!).
Navigating this challenge involves the ever-changing boundaries of billing. Initially, you build a monthly billing system, only to find that your team requires a yearly one, which seems relatively straightforward. However, complications arise when you introduce usage-based billing, incorporating weighted values akin to storage, layered on top of a quarterly plan. This complexity is further exacerbated when custom billing structures are needed for negotiated contracts, adding another layer of intricacy to the mix.
