How shall I say this diplomatically...I don't care what the vendor says...I test their water. It takes 10 seconds, 2 drips on the refractometer done. Sorry had to say that. Please, just test it. I mean you can ask and then have the tank set up for that, but I would still test the bag water to make absolutely sure IF you are not going to do traditional drip acclimation. If you are going to drip, then it doesn't matter.
If I have to buy more time because my salinity is several points off, I place the new arrival in a specimen container (
https://www.amazon.com/Lees-Convale...527734398&sr=1-1&keywords=specimen+containers) and put a small airstone in there as I wait for the water to reach the desired salinity. You can even begin to do a mini water exchange in the container half a cup at a time if you like. Keep in mind that you don't want to mix the waters, but because as I said earlier, I tank transfer this isn't a major concern to me.