1GB bundle requested but not actioned

We visited MTN for an unrelated query & the manager noticed high data usage on the primary number. He suggested a 1Gb data bundle, which we agreed to & asked the consultant to add it. It wasn't added and we noticed the Dec 2014 bill amounting to R4703.90, was debited in Jan 2015. We advised them that the bundle wasn't added and they once again did not add it and the following bill for Jan 2015 amounting to R3969.80 was debited in Feb2015. We would have been charged a low in bundle rate within the 1Gb and a reduced out-of-bundle rate, exceeding the 1Gb. Once opened a billing query, they explained that they could not find the document that requested the 1GB bundle so they cannot pass a credit. I responded that while they could not find the document, this does not mean that it wasn't requested, twice. We were willing to pay for data used at the applicable in and out of bundle rates, but being charged what they had charged us was unfair. I still have had no response.