Once paid you can not change order

This is unbelievable!!! what a bad service, I am on my way from Gauteng to Limpopo to attend a wedding. I then passed by ultra shell on the N1 where there is Steers as I am a steers fan and used to the good service I always get in Gauteng. My husband and I bought two meals just a second after he paid I asked if I can change the meal and was told once the slip is out, you cannot change your mind. I then asked if I can get my money back and I was that also cannot happen as the manager is here!!! I mean am forced to eat what I do not want because in this restaurant you cannot change your mind!!! it was better if the order was ready but it was at the same time that we just paid the order. I really do not find this as a good to your customers!!!! I am not happy!!!! I even tried calling the cellphone number on the slip many times however it just rings with no answer.....what bad service!!!!!