Time Warner Cable Continued billing for a returned modem San Antonio, Texas

Time Warner has no shame.  My beloved cousin died in early November, and I have been trying to close out her account since then.  She had a bundle package with internet, television, and a landline home phone.  We cancelled some of the service shortly after my cousin's death.  We also returned two converters at this time (I have a receipt for these returns).  We chose to retain the cable coverage till the 15th of January.  I called Time Warner on the 16th to cancel the remainder of the service.  They stated they couldn't cancel the service till the modem was returned.  I returned the modem to one of their stores on the 19th of January.  I asked the person receiving the equipment if this was all I needed to cancel the account, and he said yes.  I asked if I would be receiving a credit on the account and he said yes.  On the 10th of February, I received a bill for $81.36 still showing that the modem was not returned.  My account has a $98.07 credit without the modem and card.  I was told that the account did show that the modem was returned and that it "just had not been put into the system" and that it now had and that a credit would be issued shortly.  They further stated that nothing was owed on the bill.  On March 1st, I received a bill from a credit collection agency for the $81.36; I had received no other notification between the 10th of February and this notice.  I immediately called Time Warner and was assigned a case number.  The rep again stated that it was clear I had returned the modem, but that they would need to verify that it was in their facility.  I don't know what that has to do with anything since they noted that the modem had been returned.  He stated that I would receive a call back by the 5th of March letting me know what the status was.  I did not receive a call yesterday so in the afternoon I again called customer service.  Again I was told that it was clear that I had returned the modem.  This rep stated that the previous rep should not have told me I would receive a call by Tuesday because it would take up to 8 business days to research the situation.  I asked to speak to a Supervisor and was placed on hold for a long period of time.  When the representative came back on the line, she stated there was a backlog of calls (not surprising after looking at all the complaints) for supervisors and someone would give me a call back within 24 hours.  In one hour it will be 24 hours and not surprisingly I have not received a call.