Drinks cartons vs bottles

I worked out recently that I was spending about £67 per year on drinks cartons for my children packed lunches. I was buying a value brand so compared to the cost of some cartons this was probably cheap.  However I decided to 'splash out' on 2 small bottles of fizz. The children were happy as we don't often have fizzy drinks and I was able to re-use the bottles in their lunch boxes. I either refill them with water or squash which costs pennies, saving myself a few £s in the process.