Big Concerts South Africa
Advertising price and payable price not the same

On 2013-05-07 around 18:30, I arriving at Cape Town Stadium to attend the Bon Jovi concert.

I bought a 500ml Coke at one of the venders walking around for R15.00 The carry case was also marked R15.00

Just before the show started, venders were walking inside the stadium, selling their products. The carry case still shows R15.00, but now he asked R20.00 People around me was also upset about this.

I've send a Face Book message to Big Concerts about this, but they did not answer till now.

Is that normal practice?
Surely this is in contradiction to the Consumer Act.