Bill me for a month they shouldn't

I gave notice to cancel my account in January so February would be my last month.<br> but plans changed and I asked them to extend my account for one more month.<br> The month of March.<br> but then I still got billed for April when I shouldn't have.<br> when I called they were happy to fix it and refund me.<br> but suddenly I owed them some money from a previous invoice that they didn't debit my account for.<br> They didn't find this when I canceled my account.<br> They didn't find it when I re-instated it for one more month<br> They didn't find it when they wrongfully billed me for the month of April<br> But now that they have to pay me money back they suddenly find it!<br> <br> It's not a large amount but rather the principle of the matter.<br> I won't ever go back to them and will warn others to stay away.