He doesn't spell it out much, but he has the concept right. You are reinventing the wheel, for something you can painlessly buy cheaper than you can recreate it. Buy a used 4-tap soda machine, some bags of syrup, and the tall pressure vessels that add fizz to the water. Like buying beer in kegs, for a fraction of the price per glass than 24-can cases cost. Any prepackaged drink, the container, convenience, and shipping, costs more than the contents.
Mind you, I do understand the impulse to see if you can reverse-engineer it on your own. But don't delude yourself, or attempt to delude SWMBO, that it will save any money, or taste better. It is therapy, and a lot safer than a red convertible and a 20 YO to go with it.