Can’t add additional refund

I’m trying to add an additional refund because the customer paid for the shipping label with her own money. But the option is nowhere to be found. I contacted support 10 times but they all give the same generic answer that it should be working but it just isn’t. I wish I could add an image but I posted the same issue on the amazon forums with an image https://sellercentral.amazon.com/seller-forums/discussions/t/5f484b4f-8026-496d-b1bf-b33ca2de3900 I remember a week ago it was still available but suddenly its gone. Does someone had the same issue and know a solution?