I Accidentally subscribe to Seventeen magazine by entering a sweepstakes! Help!?

I am already subscribed to that magazine by using icoke rewards. I got the first issue of that and when i saw a contest I decided to enter it so i did. After i clicked my info it said thank you for subscribing! I didn`t even want to sub them! I tried cancelling it and it did. I thought it did anyways. Then like just today i recieved the bill through email. I was pretty sure i cancelled it. So what do i do?? I heard that writing cancel on the bill then shipping it is the best way....