I think the note was written by the sender because the handwriting looks the same as in the vague "woman in Cornwall shed" address.

My guess is that it was intended to motivate Royal Mail to figure it out by quoting what might have been their slogan at some point. Now the highlight could have been added by a post office worker as a way of saying "indeed we never fail". The sender's style of higlighting things seems rather to be adding asterisks.