Priority Review subscription charges are not refunded or prorated, as the Refund & Cancellation Policy says (/legal/refund-policy). When you cancel, every future charge stops and Priority Review stays on until the end of the month you already paid for — so you keep what you paid for.
We always fix billing mistakes. A charge made after you cancelled, a duplicate charge, and a subscription that never turned on are refunded — automatically, when our records show them — through a change request (/help/change-request). A charge for a different amount than you agreed to is checked by a person and the difference refunded.
Send a change request: billing mistakes our records prove are refunded automatically, and a person answers the rest within two business days.