I did a prime trial and then cancelled it... only to get a bill a bit later for prime. My account showed cancelled, I did not have active prime. I had to call i

I did a prime trial and then cancelled it... only to get a bill a bit later for prime. My account showed cancelled, I did not have active prime. I had to call in and the phone help explained that the auto bill option was checked... Nowhere on my side of the UI was an auto bill option shown and in what bizarro logic would auto bill stay active after cancelling?