Displaying one price and charging the customer another.

I wish to make a complaint against Featherbrooke Steers, Ruimsig. I bought for myself a King Steer snack burger. On a bill board displayed the price was shown to be R29.90 however when I asked for it came up as R30.90. I asked the cashier why the price was different from her billboard. Her being unable to help me called either the supervisor or a manageress to assist me. I asked her now as well why the price was different to the billboard displayed there she was completely unhelpful and just told me that, that was the price on the teller. She sort of was laughing about it&I asked her how it becomes my problem as the customer? I just paid the difference however it is ridiculous that a price that is clearly advertised is not the price that ends up being paid. For me it is the principle of the matter as if I was a rand short they (Steers) would be very quick to want their money but I have seen on multiple occasions that the customer must just accept whatever outcome. Steers must remember that the fewer customers the less profit&less profit means less jobs.