Priority Review is sold as a one-time purchase: one charge covers the application you buy it on and every application you send in the next 30 days. It doesn't renew, so there's nothing to cancel — unless, right after paying, you choose to keep it on (see "Can I keep Priority Review on after my 30 days?").
Where Priority Review is offered as a monthly subscription instead, the checkout says so before you enter a card: the monthly price, the first charge, that it renews on the same date every month until you cancel, and how to cancel. It only starts if you tick a separate box agreeing to the recurring charge.
Not sure which you have? Your receipt says "One-time" or "Monthly subscription", and /applicant/subscription shows any subscription on your account.
Send a change request: billing mistakes our records prove are refunded automatically, and a person answers the rest within two business days.