Same Weight and Price p/kg but charged more to mix the 2

I have been going to FLM for many years now and often go to Stoneridge or well as Waterfall due to one been close to work & other close to home. Waterfall I always seem to bang heads with them as if one takes a Tuna Pasta Salad @ R120p/kg and add another Cheese Pasta @ R120p/kg into 1 container, saving the cost a container, you get charged @ R140 p/kg?? I have discussed this with managers from Waterfall numerous times but get told it is to with codes??? Logically it makes ZERO sense to put Tuna Pasta into 1 container & Cheese Pasta into another container, stacking one's fridge with containers - whilst putting the 2 Pastas, @ the same price & weight into 1 container, but been charged more? Makes ZERO sense and is very annoying to be overcharged for the same weigh, same price p/kg in 1 container!