Bad service, buy R500 get headsets for free

Im very upset with the service i got today is bought 2 pairs of shoes today for R500 after i paid and walked away the cashier told the man behind me if he buys for more then R500 he would get free headsets why wasn't I told about this and i did pay R500 on the dot why didn't i get one and when i asked about it they acted like they didn't hear me this is not fair survive and i want and explication why i wasn't told about this and why i didn't get one when i paid R500, i want my free head. The survive was terrible.