I currently use dowflake, baking soda, and MagFlake as supplements in my tank. If you need something and can't find it locally, why not buy from these guys? They're just selling the products that Randy-Holmes Farley has tested and recomends, and pre-measuring for those that don't want to be bothered. It's a smart idea, wish I'd thought of it first...
But, before you pay to have something shipped, think about what it would cost ot get it locally. Baking soda is available anywhere - 59¢ for a small box at Publix, to $3 for 8lb. as Costco, or even bigger bags at pool supply stores. I've bought Dowflake from Pinch-a-Penny pool supply - $20 or $25 for a 50lb bag. If large quantities are an issue for you, buying smaller packages from twopart might make sense for you. (I store my bulk Dowflake and MagFlake in sealed salt buckets in the garage.) I've always picked up MagFlake through group buys in this forum.