Clarify seeking

On Thursday 27th of July i bought a 5 piece at this establishment (KFC) in Standerton infact i ordered online. The food i ordered was suppose to come with extra piece as per the adverti*****t online menu. But to my surprise when i got home the were only 5pieces. I tried to call the shop but i couldn't not reach them. I would like to know how this came about. The adverti*****t says if you spend R79 or more you get extra piece.