Plastic issue at MR Price

On the 13th November, I went to MR Price and I was not really looking for something specific and I got myself a beautiful top and a pink sports bag at Mr Price Riverside in Nelspruit. I went to the cashier and gave her my stuff and she scanned the two items and when I was about to pay I noticed that she had also scanned a plastic bag. I advised her that I do not need a plastic bag I will use the bag I purchased. She called her manager or supervisor to void the purchase. While all of this was happening the cashier next to her was telling me how cheap their plastic cost and that it was cheaper than a plastic bag at Checkers. I told this lady that what she is doing is unnecessary and unprofessional. I could not understand why she was being personal and why she was embarrassing me in front of the whole store because she was not even the one that was assisting me. I did not need a plastic bag and why am I being treated like this for not wanting a plastic bag. I left the bag and I just went to a different store in the Mall. Regardless of the price of the plastic bag being cheap at Checkers but at least cashiers at Checkers will ask you if you need a plastic. And why was this lady attacking me and she was not even the one assisting me. I ask myself how many times this happens at the MR price on a daily when it comes to the plastic issue.