As far as I can see, 18 North Bridge should be the last shop below the Balmoral Hotel (the North British Hotel until relatively recent times) as you head southwards from Princes Street, immediately before the bridge itself. I have looked at Google images and a few similar sites to try and find a picture showing the shop in more recent times. Sadly the easiest views and probably best views of the hotel are of the other façades, so these are the ones found on the net. Google maps street view seems to have no coverage of North Bridge.
ADP