Help with Selling Partner API (SP-API) needed

Hi I’m a software engineer and I’m currently developing a web application for a client who wants to pull data via the Selling Partner API (SP-API) and calculate certain KPIs based on that.

Since the SP-API offers many different “sub” APIs, I’ve found it very difficult to determine the right APIs to use. (Unfortunately the client is not familiar with the API, but only with data/reports on Seller Central)

So far I was able to pull most of the needed data via the Orders API, Reports API and Finances API.
But I have troubles getting some fees, especially FBA fees like the storage fee, long term storage fees, FBA disposal fee and the condition of a return via the API.

Additionally I have troubles with all the different possible FBA adjustments.

Is there someone who is familiar with the API and such use cases and could me point into the right direction? Help would be greatly appreciated!