Kfc sarnia overcharging

Today I went to KFC Sarnia, wasted my time waiting in the drive-thru. The menu boards listed the twister meal for R54.90, when i get to the cashier, he wants to charge me R57.90. He says there is nothing he can do as on the system it is priced as such, he called the manager and told me i rather not buy because they cant charge me what it is advertised. I found this so annoying, wasting my petrol n my time only to be told i must pay the wrong price. I had to drive another 7km to mcdonalds. Why put one price on the menu n want to charge a higher price at tillpoint n you are not apologetic about n you n simply say i would rather not buy! Listen i have been a customer for 23 yrs n never experienced this nonsense