Harrassing me for money I don't owe

I'm at a loss for words. In my lifetime, NEVER have I experienced SUCH bad service from one company. In Feb this year I went for quite a few blood tests to ampath. I paid cash for my blood tests, preferring not to use my medical aid, and left. A couple days later the lady calls and apologises. She made a mistake, and omitted to charge me for the one test. I was in a meeting and I asked to please put the one test which she didnt charge me for, through the medical aid. The next thing I know is the ENTIRE battery of tests that I took (and paid for) was then put through my medical aid. After much bother and admin on my side, it was reversed by the medical aid and the one test which she forgot to charge me for, I then paid via EFT. Since then AMPATH have been harassing me fora balance of nearly R500 which I do not owe. The cash rate is LOWER than the medical aid rate. Because THEY put through the whole battery of tests through the medical aid, A MISTAKE ON THEIR PART, their system shows that I owe this balance because of the difference in rate. I keep calling and the consultants see the mistake and they tell me they are rectifiying it. Today, I received a final letter of demand! I paid this on the day I took the tests, in the very rooms I took the tests!! The incompetence started from the lady in the rooms who missed charging for one test, then the accounts departmnet who put through to the medical aid an entire battery of tests that was already paid for, right through to the call centre staff who cannot rectify the issue on their system. God help you if there's an admin mistake with these people. They are *********** and quick to handover for money not owed to them. NEVER WILL I EVER utilise them again. From this point on, i will only use Pathcare. If this is not rectified IMMEDIATELY, I will be posting my story on FB and TikTok. Please note, it is *****ulent to ****** money not owed to you. Train your staff. Fix this.