I canceled a magazine after signing up with the "Bill Me Later" option. Am I obligated to pay?

I signed up for GQ, Esquire and Details online and used the "Bill Me Later" option for payment so I could compare the 3 magazines before deciding which one to go with. I canceled all but Esquire. Today I get another GQ and Details in my mail box. I canceled my subscriptions to those right after I got the starting issues. I then checked on the customer service website for the two magazines and it says I have an outstanding balance. What happens if I don't pay for them? Is there anything that they can actually do? If there is I want to make sure that I pay them; but I figure that's why they offer a "Bill Me Later" option, so people can try out the magazine before committing. Thanks!