Poor service and harassment from jet

I have has a jet account for a number of years and had no issues with them .on the 26th of December I went to the Hemmingways mall branch and purchased for just over R6000 which depleted my credit. At the till I had to pay in R900 which i did .After I completed the purchase I then paid R2000 on my account .the store manager and staff then asked to take pictures of us with the slip as to show the regional manager they had done well On about the 7th of January I received a mail from jet suggesting I need to make a payment of R400 as my account was in the areas I ignored this as I knew I had paid R2000 on the 26th of December .I thought maybe with the public holidays that the credit department was behind. On the 10th of January I received a call from a collections person asking me to make a payment arrangement with them as I owed R500 on my account. I disagreed and told them I owed approximately R4000 on my account but had paid the R2000 on the 26th of December they said according to them I was never in there branch to make a purchase or payments and all I owed was R500 On the 15th of January I then again went to the hemmingways mall branch and they remembered me and confirmed onthe system there was no transactions for me on the account. I paid the full amount owing on their system and asked the manager to please look into the issue as I am not a ***** and wanted to pay my account properly. After some time on the 25th of January I received another collection call and after explaining my issue and emailing the payment slip they promised to sort this out.againon the 29th I received a collection call and again sent the proof of payment On the 30th again another collection call On the 31st of January I called complaining of the harassment from collection people after all the the mails to provide them with proof of payment and promises were **** to sort out my issue Every single day sometimes up to 3 times a day I still get collection calls I am really frustrated and in total disbelief that a company cand have such poor accounting skills and have no communication from one department to the other