After our email exchange where I tried using your Chameleon protocol I think I have a working hypothesis as to what’s going on.
Although it worked when I changed protocol I think this was just because I hadn’t accessed Amazon since the day before.
I think that if I go to any Amazon site and use my account - UK, Italy, Germany - they log my location. If I then switch on VPN, I suspect they apply the location they just logged to my account, not to anything on my computer.
So at the moment I think the secret is to only access Amazon via VPN, and not leave any Amazon tabs open in the background, if you’re going to want to use location locked services.
I’ll keep monitoring the pattern of what works and what doesn’t and when, and will report back if I find something to disprove this hypothesis.