Tracker Connect - Pathetic Service

Sold my vehicle and phoned to cancel my tracker unit. Informed by Tracker that it is a 3 year contract and that I need to pay the remainder of the contract. Had no problem with it, and received a letter indicating the amount with all the steps to be taken for the cancellation. Followed every step, paid the outstanding amount and sent proof of payment to the email on the cancellation letter. Received an invoice from Tracker 2 days ago indicating I must still pay in advance for the new month. Phoned Tracker to sort out the issue, only to be informed that I should have phoned to arrange cancellation after payment was made!!! I don't understand why I should have phoned to cancel as the proof of payment was sent to the indicated email address on the cancellation letter with the correct reference and amount!!!!!! Pathetic service. I will dispute any deductions from Tracker.