Lego 20% off all LEGO - Not true

I received a sms from Toy Kingdom - Get 20% OFF ALL LEGO betwen 24 April- 3 May! It was also advertised on radio and there are signs in store. I even got brochure in store. As it is my sons birthday next month I especially went to Toy Kingdom in Canal Walk, to get him some Lego. I decided on a Star Wars set priced @ R 899.00. When I got to the cashier I was charged R799.00. After I paid, I checked the slip and realised it was not 20% off. 20% is R 180.00 off and I only got R100.00 off. I asked the cashier and she confirmed it's 20%, but the tills are pre-set to deduct the discount of 20%. I explained that R100.00 is not 20%. She called the manager, same story. Sorry mam, the till is pre-set, we can't change it. Do you want to return the lego? That is not the point! It's false advertising! They say 20% off, but you don't get 20% off!! This is totally unacceptable!