Did you try reaching out to executive customer relations, either through ajassy@amazon.com or jeff@amazon.com? (FWIW Jeff's address still works despite him stepping down 2 years ago)
Always had a very quick response and resolution whenever I had an issue that was beyond the ability of "normal" customer service to resolve.
This approach is incredibly effective, and will almost certainly guarantee one of the infamous "question mark" emails gets sent out. There's a whole process built around question mark emails that drags in a bunch of senior staff to ensure things get resolved.
You can read about it here: https://www.inc.com/business-insider/amazon-founder-ceo-jeff...
I've been on the receiving end of an ajassy question mark email once. That was an interesting way to spend a few days. The customer was complaining about slow uploads to the aws service I worked on. All I had to go from was their country, via some guesswork. They did at least mention which region it was they were having issues with.
It took a bit of effort but I was able to prove from service logs, to a reasonable degree, that the problem was with their ISP. All I had to go from was what country they were in, didn't know their ISP, which added to the fun.
I was in the middle of getting scammed by a company out of Lithuania when I mentioned that I was going to turn this information over to a reporter at a company I work for, and the tone of the CS person changed from "I can't help you" to "yes, we will give you an immediate and complete refund."
The only mitigation I've found is to make multiple orders separating out heavy/bulky/liquid/light/fragile items, shipping cutoffs permitting. But of course this is less likely to work with Amazon where they coalesce different orders. (The latest incident was due to forgetting to do this, which should be their job, for them)