Invalid wi codes cause humiliation at till point.!!

I complained about the new improved taste of Nescafe coffee it tastes awful, even the smell is not the same, So a Nestle consultant phoned me, while on the phone he sent me 2 x R150 wi codes and 2 x R30 wi codes, while on the line I had to confirm I had received the codes, witch I did get. On Sunday I go to my local Checkers and while at the counter I give my codes, but only ONE code of R150 worked, I stood their at the till like such a fool, very imbatrased and hiliated while the teller was unable to access the other 3 codes, knowing I had just received these codes, I did not take extra money with me, looking like a fool, because 3 of the codes did not work.... I had no way to verify why they were not working Nestle please ensure you do not cause further humiliation while giving invalid wi codes it is not fair. !!