The powedered sugar from the store works fine for mite treatment. It doesn't work so well for feeding. In fact you can use flour for mite treatment if you want.
If you are going to FEED "powdered sugar" then I'd get the fine pure sugar you're talking about. The bees will eat it much more readily than granulated. And, as mentioned, you can use a blender on your regular granuated to make your own in small quantities.