Walmart Family Mobile aka Walmart Family Mobile Phone Service Continued getting service bills after service was cancelled Nationwide

Shortly before moving to Tennessee from Texas, I had been forced to close the bank account that was linked as a direct payment method to my phone bill with Walmart Family Mobile, as the account I had was will a local credit union.  I had opened a new account with a different (national) bank and had updated my information online to reflect the new changes before paying my bill.  

In December when I paid my bill with Walmart family mobile, I even received a text on my phone saying that the payment had been processed.  Little did I know that the Walmart Family Mobile system had not updated properly, and the payment was pulled from my closed account.  WFM then put a late notice on my phone, cut off my service before payment was due, and doubled my bill to $64.02.  I could not even use my phone to call a representative, but instead had to use my husband's.  

After contacting WFM, I paid the "overdue" bill (still before the actual payment date) and told them to cancel my service.  I was told that I would have ONE more payment to make three different times by the representative.  My service was cut off two weeks before the date quoted to me as my last day of service, but as I planned on getting a new provider I didn't bother reporting it.  I made my last payment of $39.72 on January 13th, which is reflected in my bank account.  

On March 10th I started receiving letters from Walmart Family Mobile that I was past due to the amount of $47.69.  The charges on the bill are listed as starting on February 7, a full month after my service was closed.  Upon contacting the 800 number provided on the letters, I was informed by the representative that the bill was for the previous month of service, which makes no sense as I had no service at that time.  Upon mentioning that my service was cut off early, the representative responded that "since you didn't report it, at the time then your complaint is no longer valid." How could I report that I had no service when, in fact, I had no service? The representative did nothing except insist that I still owed money.  I am legitimately concerned that this is going to continue to be a viciouscycle, and that I will just keep receiving letters saying that I didn't make my "final" payment.