Lancet Account problems

Their account department is a complete joke. For some inexplicable reason, known only to Lancet, I have two accounts with them (M***7 & M***0). I paid an amount of R1,805.50 to Lancet, against both accounts (the sum of R1,415.00 & R390.50), on the 12th of March 2020. Since then I have had telephone calls, SMS's and emails harassing me about the amount of R390.50 they say hasn't been paid. They continually ask that I provide proof of payment which I have refused to do unless they pay me for doing so (I'm self employed and my time is my livelihood). Today they have stated "Please be advised that no payment has been received on our side" when referring to the R390.50 - an obvious lie as they are silent about the other amount of R1,415.00. How are companies with such *********** account departments still in existence?