I took the car to Ronnies for an adblue refill

I took the car to Ronnies for an adblue refill.I was told that the whole adblue system needs to be replaced and that will cost R36,900.When I got home,the adblue was working well.I had already paid R15,000 deposit to them for the part that will be ordered.I told them I no longer need the part as the car is working fine.They do not want to give my refund,saying that the parts are not returnable.Melony Boucher was the service advisor and she has not been very helpful and she was rude to me.