Disgusting service

On the 4thMarch 2023 I paid R1500 towards my account which left me with a credit balance of R10-20 than on the 4th April I received a statement showing that I was charged R425 interest on the 1st April I phoned homechoice and a consultant advised me that because I did not request a paid up letter therefore I was charged interest on a credit balance this is something that I have never heard off my account was paid up on the 4th March and now they say I owe R425 even though I didn't purchase anything after the 4th March 2023 I have sent several emails and no one can assist me this is terrible service I would like someone from their accounts department to contact me about this interest charge. I have never heard of people being charged interest for having a credit balance disgusting service