Struggling to get a letter from my employer, what else can I do?

I need a letter explaining 2 categories in my paystubs (Bon (bonus) and Ex Gratia (I believe it’s them fixing underpaying me)).

I requested this letter 2 weeks ago and it’s been just back and forth with no result or even finding a person who can take responsibility. My job is fully remote and the company is huge and super bureaucratic. I can reach out to people only through email which is easy to ignore.

What else can I do to get this letter?

I live in California