Bad attitude and unprofessional cashier

I never meet a rude cashier in my inter shopping life ever, I went to Shell Pinegowrie yesterday at past 6pm to get a grand pa for a very strong terrible headache since i got flue during the day at work. I was so desperate and this was the only nearest place i could buy a grand pa. As i get in a store i was told that cashiers are exchanging shifts but i patiently waited for them to switch. The only money i had in me was R50.00 i the lady looked at me straight in my eye and tell me with very bad attitude she cant sell me grand pa with R50.00 as if i was taking a chance. I left the store and one of the patrol attendant offered me R3.00 which unfortunately was taken from were he placed it by someone else. So maybe this shell or this lady had enough of our money and inhumane since i was suffering a headache she should have at list tried to help me regardless of how much i had. Please deal with this lady as one day she will make a worse mistake than this which will lead to loosing of business