Pay-per-month memberships include a minimum commitment period of 3, 6 or 12 months depending on which plan you choose.
If you change your mind and wish to cancel before your commitment period has ended, you can do so by downgrading to a shorter plan and paying the difference between the monthly fees.
For example, if you purchase the 12 Month Plan and wish to cancel after 3 months, you will be asked to pay the difference between the 12 Month ($19.99/month) and 3 Month fees ($39.99/month) in order to exit the contract.