Because, Curuinor, post people are about the bravest people in the world. They may err, but not often.
By that time, they wil have pliable wings structured of ultra light fabric on a frame of ultra light metal and fly on manpower only.
As has been written by the clairvoyant writer G.R.R. Martin in his little book "Windhaven".

Windhaven

Of course it would take long for the peanut butter to get there,
but when we will be 104 we will not mind a day more or less.

I would be willing to reconsider the bet on this condition. One pot of peanut butter, payed for by the sender and delivered on wings.
Whoever wins the bet hires a wind postman.