I had exactly the same issue with Anthropic API. It was only $15, but I was so annoyed when they just decided that they'll take my money for free. If it's really the law as some people state, it's a stupid law.
I think my Zalando gift cards expire after 4 years.
Fal.ai does the same thing.
It's pretty much a universal API credit policy at this point. I'm not sure if this legitimately escapes the prepaid gift card requirements or if the providers see nuance where there might not be any.