I get the feeling from my latest attempt to buy my grandson's Xmas present that if you are a previous customer of Amazon it's Amazon Prime or not at all. (After three attempts I chose Waterstones)
I had just this, unclear website trail and I joined Prime by mistake. I opted out straight away, and the goods didn't turn up! Eventually got them, but now stear well clear.
The guy is now the richest person in the world and his company tries everything to avoid paying tax in various countries so I won't touch Amason now. Local shops for local people I say... Or they will all disappear.