You ain't getting nothing.

So today I went to my Dischem store at Promenade as I always do, I had to buy few items using my voucher. So now as I told the cashier I'm paying with the voucher and she was okay with it. She asked for my voucher number and later asks me why I'm not using the full balance of the voucher I told her I'm going to and she told me that she already completed the sale and I asked her what about the rest of the stuff on my trolley and she told me that I should have put everything on the counter and should have known she's going to complete the sale when she asked for the voucher number and now I'm only getting what she rang for and I asked her what about the rest of the purchase that I was still taking out of my basket, she called one of the males in the store and I was told that it was my fault I was supposed to have put everything in the counter and I'm responsible for the cashier making a mistake, then another male was called and he told me the same thing so I asked him if the cashier makes a mistake why is it my fault and I was told I sent have known she wants completing the sale. I mean I don't work for them how was I supposed to know she was completing the whole thing and they offered me a voucher for what they put on the sale. I told them I want the full balance because they acknowledge that it was a cashier's mistake so why am I paying for that. It was a bit of a scene and customers were looking at me, I felt embarrassed but I told him I'm not going to take that amount they gave me and I left my purchase there because they wanted me to take it and told me that there's nothing they can do to assist me, and yes both the cashier and 2 gentlemen acknowledged that it is the cashier's fault and they are not going to reimburse me. I was very shocked by this because this is not the first time I bought something at Dischem using a voucher and I never had an experience like that. I buy my cosmetics there every month and I never had such treatment ever. I am beyond disappointed.