I requested a cancellation but my confirmation was not complete

I called and spoke for over an hour on the phone attempting to cancel my services after over 5 years of service. The agent first told me the cancellation date was made for the final day of my billing cycle then she said she had made a mistake and had cancelled it effective immediately, she then transferred me to another agent who assured me he would fix it and walked me through the fix assuring me that my cancellation would be scheduled for the final day of the billing period for which I had already paid and that no further bill would be issued. He also gave me a $20 credit he said apologizing for the mistake of the previous agent. I then received an email stating my cancellation request was received however the date of last service was left blank. Furthermore I then received a new billing statement for another month of service covering both a period of time that I had already paid for service and a period of time falling after the requested cancellation date. How can I confirm the cancellation of services date or last day or service and how I might confirm a redaction of the additional bill?