False Advertising

They Advertised Tyres for R100 and as soon as I selected the tyre. The Price displayed R0.00 and they showed 7 in stock. So I ordered 5 Free Tyres. Upon checkout there was a calculate delivery cost witch I Entered my address and the Price for delivery of the free tyres were R100 witch I gladly paid. I even received confirmation of my Order and now they tell me that it was a mistake etc why advertise something if it is not supposed to be there