I found out how to cancel my Premium Care purely by accident. (It's not like Samsung is going to make it easy). I found it by accident while using the Shop Samsung app and signed into my account. Once you're logged in, go to "My Orders" on the left hand side, in the drop down menu. Once you're there, you'll see at the top left, "My Orders" and next to that, at the top right, you'll see "Subscriptions". Click on Subscriptions and it should show your Premium Care subscriptions. You should be able to see an option that says "Cancel my Subscription", then click cancel. It may take a day or two to show up as cancelled but it will. Good Luck!!
It's crazy because last week same thing happened to me. I was buying a new phone and did everything you're saying. I canceled it and received a confirmation email. Thank you for your answer though!