Poor service

I opened an account earlier this year with truworths. My credit was R500 and I used that all up. My first installment was in March and I made an immediate payment of R210 instead of R204 on the 28 of March via EFT. These people have been calling me non stop and sending me messages and I keep on telling them that I did pay and I've been sending the my proof of payment. I do not know what to do now Im just frustrated by their service and getting endless and unnecessary calls. This will ruin my credit score and it's not fair because clearly they cannot do the work.