Bad service

I am struggling for 2 months with a purchase a made at macro online. I went on macro online shopping, purchased a tv box and paid with my macro credit card, the next day I paid the amount in full. Now macro store emailed me, sms me that I am in arrears on my monthly payments and it will be bad for my credit record and they charge me interest. I have been in macro store at Brackenfell 3 times to explain and show them my invoice that I paid the full amount, they can't help me, they gave me number I must phone that I tried to phone and explain they can't help me, the lady on the phone spoke with food in her mouth that I can hardly understand anything, the 3de attempt they said I must email the invoice, which I also emailed 4 times already, still my account is not updated and still get sms and emails that I have to pay because it will have a negative impact on me. What must I do pay twice now with interest? No one can help me