I think you're in the US so I don't know if this is an option there, but I pretty much always used the ready-to-feed cartons (the like UHT kind that you can store at room temp and just open and pour into a sterile bottle to feed). We did go through a phase of trying to use the powder and make them up fresh with boiled water in a thermos for a time, but that was too much of a pain! After enough times trying to cool down a bottle in bathroom sink of cold water while dancing around with a crying hungry baby, I decided the money saved was not worth the hassle. Here they come in 200ml cartons so if your LO won't drink all of that (or wants more and you have to open another), you will waste some, which again is an added expense. But it was worth it to me.