Secondly, the date on the far right of the cover gives every impression of saying 1806, hence my request for a scan of the inside contents.
That, I noticed as well. But the '6' looks nothing like the '80' and is set differently.
My next thought was the distance. To be honest, the '1' may look more like the '80' but may also not have been written at the same time. 80 miles appears too short and 180 way too long. More importantly, there was a new uniform postage rate system. So, distance would be irrelevant.
Just above the stamp is a note (I think) March 20 41 that would fit the late use of a Penny Black, just after it was replaced by the Penny Red.
I think the recipient would have been asked to pay the "postage due" charge of 1d, but hopefully somebody can confirm this.
After affixing a stamp, does the '2' denote the postage or the amount due?
So 2 - 1 = 1 or 1 + 2 = 3? 2d was the second weight step tariff, 3d was no letter rate in the early 1840s.
Still, I remain suspicious about the lack of 'tying' of the stamp.