There have been at least two debates on the subject in different threads in recent weeks on this site.
You may like to have a look here: click here
and also here:
Where the FE posted:
"I understand that it currently costs the Post Office £7 to deliver each first class letter to Orkney"
So on the assumption that it is true, Royal Mail only don't add a surcharge to you because they do it to all of us so you don't have to pay extra. If they ever got their way and could charge different prices for different distances the story would change.