cashier is judging how much we spend

I'm a black girl going with my white boyfriend while paying at the till I ask my boyfriend to buy me airtime and he say o so I ask da ccashier to plz ring R12 airtime when we walking out she is gossiping with the lady who pack's saying a white man can afford R12 airtime I don't understand why we should be judged on what we buy or who buys it and when I confront her she says yes I said I thought he was going to buy you R200 airtime why just because it a white man he must buy me R200 airtime