28% of people surveyed unintentionally signed up for Prime - https://www.which.co.uk/news/2019/07/accidentally-signed-up-to-amazon-prime-youre-not-alone/I inadvertently signed up to Prime and didn't know until I received my Visa statement. I promptly stopped payment and took all details of my CCard off the account. I am not using Amazon anymore because I do not know how I set it up.
Could there be one of those boxes that you must un tick rather than tick?
You’re not alone. To be honest I don’t know how Amazon have been allowed away with it for so long. They take very deliberate measures on their site to trick you into signing up to it. I imagine they succeed in tricking older less tech savvy users particularly effectively.
FWIW they’re very quick to issue a refund if you contact them to say you didn’t want the service.
Note you do get the Amazon TV service with Prime, and there’s some good content on there, the price isn’t so bad if you use this.