> it took several conversations with support staff, and after a few rounds of "free prime extensions", they stopped the shipments via amazon.

That's what it takes. IIRC, they'll only consider deprioritizing AMZL if you've had 3 misdeliveries (by their reckoning) in the last 6 months. I haven't had the time to basically go to war with them to get them to stop, I just shifted my shopping habits instead.