Price discrepancy

I went to the Total Energies garage, corner of Veld- and Vlei Streets, Kempton Park to purchase a bag of Charcoal. The price on the shelf indicated R129.00 but when the cashier scanned the barcode the price changed to R189.00. I indicated the discrepancy to the cashier, however, she just dismissed it. I returned the bag to the shelf to reassure myself of the discrepancy. As far as I know - the price displayed is the price you pay.