Some companies require you to submit cancellation requests a day or two before the start of the first billing period. Personally, I always cancel 7 day trials on the 3rd day. I still get the full 7 days free trial, but at least I know the company will get the cancellation request in time. I never cancel free trials on the last day.
If I were you, I'd contact Samsung or whichever company your subscription is now with and explain the issue. Hopefully, they'll issue a refund. More importantly, though, is you need to confirm it's actually cancelled otherwise, you'll continue to get charged.
