Audible charge through Apple but no credits

I have been charged monthly for my husband’s audible account which I set up but have only just found out from him that he hasn’t received any credits in this time. I contacted Apple and they have refunded the last two months worth but I can’t get the rest of the money back which comes to at least £100. Audible will not look into it as it is an issue with Apple. Does anyone know what else I can do? It looks like I’m not the first person to have this problem.