So just put in bags the amount of salt you need to mix up enough saltwater you need for a water change, I didn't mean make a great barrel full, all you are doing is bagging the salt yourself rather than paying julian sprungs to do it for you, then what you save can be better spent on corals or fish.
You need to buy a small tub of salt (your choice of brand but buy what matches your parameters the closest) and a few bags to hold the salt in such as zip lock bags or similar, Compare that with the price of the pre bagged stuff, plus im sure you cannot pick which brand of salt you can have with the pre bagged stuff