Charged for cancelled contract

I cancelled a month to month home data bundle on the 27th February at the store. The router has been returned. Today you decide to send me this email: "Y'ello! Your services will stopped in 1 day and you'll be reported to a credit bureau if MTN doesn't receive payment of R 332.5. Pay in-store or via your bank. and use billing profile BA*** as a reference. You can find the banking details on your invoice." What's going going on? Why am I being charged for something which I cancelled.