Contract upgrade gone wrong

So I call in to cancel my contact after i finished paying off... I get convinced to to upgrade getting told the only change will be from airtime to data... Saturday when it come trough I am give an open line I call in to reverse it the said they it can only be done next month now I can call because I will definitely over the limite secondly u receive my invoice... I cancelled my insurance and now I have to pay it because of a system error and now that has become my problem