We've only gotten the salt water from our LFS when we setup the 75 (total volume with sump ~100g) and 150 (total volume with sump/refugium ~ 190g).
We are on well water and the pressure is not the greatest. It would take a while to make enough water for the whole setups so we took the easy way out. they actually delivered the water for the larger setup.
Here in Montréal,lfs dont sell premix water,so we do our own! And another thing ,we cannot bring back fish to the lfs for store credit only for free and refund does not exist!LOL
I use natural seawater from the pet store (5 gallons) jugs. I only have a 47 gallons tank. So, the cost isnt much of a factor. More than makes up for the simple water change with no mixing.