A Golden Argosy

This is a Novelette by Fred M White. The story revolves around Nelly and Margaret Boulton. Margaret Boulton works for Nelly's father. Miss Selina Wakefield decides to send Nelly away to hurt her father. Before his death, he tasks Margaret Boulton to deliver a letter to Nelly in London. Though they both do not know where she lives in London. Will Margaret Boulton deliver the letter? What secret does it contain?