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.
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.