Poor Service

I buy all my groceries at Pick n Pay Darwin Road, Kraaifontein. I bought a bottle of cooldrink last week and took a bottle of Twizza iron brew, however when I got to the till noticed that I took Twizza naartjie. I didn't want to buy the naartjie as I still had a little bit left in my fridge and I wanted something else for a change. When I saw it at the till I thought its weird because I am sure I took the iron brew however paid for the naartjie because the shop was busy and I wanted to go home. Sunday, I decided to go back and change it for iron brew. When I asked the lady if I could change it, she called the lady in charge who told me that they cannot change it because the bottle is open? This was a surprise to me because I haven't touched it. Then I remembered that there was a guy packing the shelf and I was wondering whether he took a sip and put the bottle in my trolley when I turned my back to him. This forced me to buy another bottle. This is not acceptable to me especially because I regularly buy there (always every day). I am sure they can afford to change the cooldrink of just R14.99 for me? I also decided not to go there anymore because of this.