Receiving invoices after contract cancellation

I went to Vodashop at Airways Park and requested for the contract to be cancelled. The gentleman called cancellations and handed the phone to me to confirm my details for security purposes. The contract was cancelled and the line switched off on the 31st of March. An invoice arrived on the 14th of April for R210.01 for a 3G Laptop device. I immediately called Percy a Vodacom cancellation who advised that it is impossible for me to have received this invoice. I called accounts this morning and they told me that the contract has been cancelled and also that they don't understand that I have received an invoice, but if I did I need to make sure that I pay the invoice..... HOW THE HELL do you get to this......