Union Square is not far from the actual Balenciaga NY store in Chelsea! It is 542 West. 22nd St. Union Sq. is 14th St. You can walk to the store! Saks Fifth is the opposite direction around East 53rd and 5th I think -- you'll need to take the train or bus to get there.
I've been to Union Sq. in SF and the high end stores are within walking distance of each other. I want to zoom into the map to refresh my memory. The other good news is they are all located on a flat area so you won't get tired walking around! Let me look at the map again.