How do I cancel my subscription?

I have always paid for my subscription via Steam, but this past time I tried to do that, it gave me an error code for the first time ever, so I was forced to used a Credit Card. I swear that I selected "one time" just like I always do via Steam, but apparently it's planning to rebill me on the 10th. I have no idea how to cancel it because in the past 5 years I've never had to deal with this.