Edgars trick customers to buy on 6 Mnths Plan interest free but put them then on 12 Mnths Plan with interset

Previously, in the first half of 2015, I had a problem with Edgars that I had changed my 6 months account plan to a 12 months plan and then later requested to change it back to a 6 months plan that they initially declined but then eventually after numerous costly telephone calls approved. As I stopped buying from Edgars I received since July 2015 a -R0.05 statement every month that clearly states that I am on a 6 MONTHS PLAN. Now that I started to buy at Edgars again I realised that my instalment due is much too low for a 6 months plan and when I enquire about that at an Edgars store they confirm that according to the system I am in fact on a 12 month plan. So now I don’t know what to do anymore. How can the same Edcon system on the one hand send me monthly statements showing that I am on a 6 MONTHS PLAN and the same system shows on their monitors that I am in fact on a 12 MONTHS PLAN and also calculate my instalments according to a 12 MONTHS PLAN with interest. I feel they ******* me into buying from them again with the 6 MONTHS interest free note on the statements. So I refuse to pay any interest and demand that they rectify their system to completely work on a 6 MONTHS PLAN.