I'm already investigating where the key gets lowercased, but maybe someone can help. It's very confusing, because in prodAdv.js it is still uppercased.
Never mind, after opening the issue I found out that I have to set the options in createProdAdvClient and not in the call method.
Or if I want to set it using the call method, it needs to be upper cased.
I'm setting the "region" option to "DE" like this
and I expect the request being sent to Amazon to look like this
but what actually happens is
I'm already investigating where the key gets lowercased, but maybe someone can help. It's very confusing, because in prodAdv.js it is still uppercased.