Although a first class stamp was placed on each letter, it did not cover the total postage cost, so they were held at the sorting office by Royal Mail.
She was able to quantify her following by the number of newspapers who bought her columns -- and by how many readers spent money on a stamp to mail her a letter.
These consisted of a piece of wood, hollowed out so that they could hold a small bottle in which a letter was placed, along with instructions for the finder to post it and a penny for a stamp.