Ask HN: Best practices for a Fee service

We are planning to overhaul our fee charging services as they are fairly simple and thus require lots of manual intervention, especially around a correction. We calculate fees daily on an accounts balance, total and post at the end of the month. We have fairly elaborate promotions and fee structures per group of clients.

I am sure however this is a well explored space with many companies (Telcos etc) that have elaborate fee calculation mechanisms. I am especially interested in how people handle modifications / corrections to fees that may have already been applied in the past - how are these re-calculated; how are results displayed back to the customer is a way that they can understand.

I of course have some ideas around this (defining a directed graph of FeedCalculator objects with customer-friendly descriptions; current fee stream vs amended fee stream...) but would love to get some insights from people who have built out such services.

Many thanks