Terrible service from an employee of Fish and Chips City

The service I got for the Fish and Chips City in Eyethu shopping centre was ridiculous. We bought food and then realised that we needed a combo that was advertised on the marketing board from the shop, To our understanding it was just a matter of topping up money because what we bought initially was R138 and the combo we wanted to change to was R150. The guy we spoke to there refused to change our order and his reason didn't make sense that the system doesn't allow it. He told us that an order can not be changed. Not sure how the process works but as a fish-and-chips employee he should have known how to help us change our order. Initially, we bought 2 small hakes, Medium chips, and a small Vienna. The combo we wanted to change to had the same thing we bought but they just had to include 2 more small hakes. We agreed that we were going to pay the difference. The guy was very unprofessional he ended up shouting at me, talking as if I'm a child. He does not qualify to work with customers. I was very disappointed with his behavior. When we were walking out I noticed that he wanted to follow us and the ladies she was working with had to stop him.