Being harassed by RCS on a closed Jet account

I have closed my JET account in January 2021 at the Cresta branch. The store clerk gave me my receipt showing a R0 balance, but did not issue me with a settlement letter, she told me to destroy the card, which I did. In February I heard nothing from Jet, so I assumed all was well and the account was closed. Since March I have been receiving calls from RCS telling me I have an outstanding balance because the account was not closed, as no settlement letter was requested by the store clerk or myself in January. If RCS changed the process when they took over the Jet accounts, they should have giving the Jet staff training, why should this now become my problem? I paid in full, to the last sent what I owed Jet/Edcon. Just by the way, the telephone number for the so-called Jet accounts department goes unanswered, the emails doesn't get answered. One of RCS's collection staff gave me a whatsup number, after 13 days they at least opened the whatsup just to come back with a auto response telling me to get a settlement letter, SO WHAT WILL I BE SETTLING. My account was closed and settled in full in January already. Is RCS using this as a money making ****??