Ridiculous covid test

I have been harrased by Lancet laboratories for covid test that came negative the amount was 850 but they exploited and charged me 1280 of which I did pay in installments.Now they handed the petty account to debt collectors who ask me to pay when i ask how much is due they say oh i see you already paid but that does stop them from harrasing me with calls daily for something that i paid.I fail to under why must one pay over 8501 as the test was 850?? They are such horrible people to deal with as they are not professional at all and exploit people who genuely pay what's due to them