If you only plan to use Poll Everywhere for a single poll on a single day, you still need to subscribe to a monthly plan, even if it is only for one month. You will be charged the monthly fee for your first month, even if you downgrade to the Free plan before the month is over. If the paid subscription plan continues beyond one month without downgrading to Free, you are responsible for the next month's account charges also, whether or not you use the service during that time.
Why do we ask you to sign up for a monthly plan? Essentially, it's more economical to provide services to you the entire month, so we do. Also, most customers prefer to have their premium features available for testing well before the event, and for reporting after the event.
Our customer support efforts don't differ much for one-time-event users and frequent, long-term users. Everyone signs on for a monthly subscription, but you can downgrade at any time to avoid being charged for the second month (and subsequent months). We don't know anyone else providing this kind of service for less money, so we say: enjoy your month. And downgrade to Free when you're ready to stop paying for premium features.