Send a change request (/help/change-request) and choose "I was charged after I cancelled my subscription". Give the receipt number of that charge and your e-mail.
If our records show the charge was made after your cancellation, it's refunded in full, automatically, and you're told straight away.
One charge can legitimately follow a cancellation: if the month's charge was already authorised when you cancelled, it completes and is the last — the cancel screen and e-mail tell you when this applies. A person checks any charge the records don't settle.
Something not right with a charge?
Send a change request: billing mistakes our records prove are refunded automatically, and a person answers the rest within two business days.