TF???

I bought an Iphone 5 USB cable a couple of months ago. This being the 5 USB charger that I have had to replace. The consultant that had helped me with the purchase assured me that I would not go wrong with the USB that he recommended and charged the USB with my MTN account payment and only handed me the speedpoint slip. Today, I went back to MTN Hillcrest since my charger had stopped working without any physical damage, this being confirmed by Geneveive. She requested a slip/tax invoice, rightfully so and I explained that I had not received neither but was willing to bring my bank statement as proof that I had purchased the USB and it would also give her dates to trace the purchase. She refused stating that they won\t use that and it is not a matter of that they can't but just won't. She then proceeded to be condescending to me in front of other customers and she smuggishly asked if I needed the customer care line number should I wish to complain. She also still went on to discuss me with other customers as I proceeded to leave the store. This kind of service is UNACCEPTABLE! Kindly advise on how we can sort out the matter with regards to my charger