This is a rip off. Bettas can adjust to most type of water and there's no need to buy something that comes out of your tap for free or almost free.
You can't just buy water and fill your tanks now after your fish have acclimated to your regular tap water. If you do it at once, you can kill them. Is there any reason why you want to use store bought water?
Even if you adjust them slowly to the store water, and then one day you need to do emergency large water changes, and you run out of store water, then you can't do it using your tap water because you may shock your fish.
If you mix it with tap water, you need to be precise how much of each you mix each time you do a water change and this is a lot of work and in majority of cases unnecessary.
If your water is hard/alkaline, and you want to make it softer/more acidic for the bettas, then buy dry almond leaves. It will make the water brownish because of the tannins, but that's the betta's natural habitat and it releases minerals that are beneficial to them.
