Incompetence or purposeful overcharge?

Checkers CRESTA - (One for R 72, Two for R 99 etc.). Showing one price and charging a higher price - AGAIN! This has happened many times over the past year or two and twice in a row this February/March. Cresta is convenient and has a good selection and quality of products. But being overcharged every other time I shop there is not acceptable! Some other Checkers stores, less convenient, seem to have less choice of products and poorer quality - perhaps they just sell specials a lot quicker? Does it pay to annoy, overcharge and waste customers time?