Overcharging

I bought 3 tins of Weighless peaches marked R 16.99, which is around the price I usually pay. Before leaving the store I checked my till slip to find I have been charged R 21.99. When I queried it, they got the price off the shelf and offered to re-fund me the difference. When I said I wanted to lay an official complaint, as R 5.00 per item is outrageous, the story I was given was that sometimes when there are price increases the system updates before the staff have time to change prices on the shelf!!! So this is even more outrageous...R 5.00 hike in price on a tin of fruit??? A 30% increase on one line item in one day??? Being ********** twice at the same time is just shocking.