Despicable customer service

I thought I would try a burger there. I asked for mayonnaise on my burger and was told it comes with mayonnaise. When I received the order it tasted like processed fish and there was no may. On collection of my order a guy from the kitchen asked for my slip which I showed him he said it's not the right slip even though it's the exact slip given to me by the cashier. The cashier didn't bother to intervene. The guy just walked away without saying anything. I tasted the burger and it was not nice. I didn't see any mayonnaise and asked the cashier for mayonnaise. The cashier insisted I open the burger to reveal if there was mayo. She claims there was but I saw none. This was a terrible customer service experience. I will not go there again as the service is a joke and the food is terrible.