Charged for plastic bag I didn't want

I did some shopping at Pick n Pay Victory Park last Sunday - as usual I was asked whether I want a plastic bag - I of course said no because I never buy plastic bags. However, on checking my slip later I saw I was charged 46 cents for a bag. I wrote to Pick n Pay about this the same day (24th) but so far I have not heard a word. I called this THEFT in my complaint last week, and I still call it theft. Actually, I'm not surprised I haven't heard from Pick n Pay, because how do you explain to customers that staff are told to charge customers for bags even if the bags are not supp****?