Thank you for sending Mary Touzeau's letter - very interesting! The words on the outside sealed section of the envelope look like French words (although she has written clearly and in perfect English) I can only guess like you, are referring to the carrying of the letter, cost of 5d, perhaps "le jeune" may mean "fast"?
I found a lengthy article about the Early Postal Service to the Channel Islands on theislandwiki.org website.
The surname "Leaver" is not one I have on my database, although I do have some Le Lievre connections to Touzeau. Perhaps Leaver may be an anglicized version of Le Lievre?
I'd suspect that Mrs Leaver would have been a child of one of Mary (nee de Grave) Touzeau's five known siblings. Research of her ancestry is limited to that carried out by another researcher through the Huguenot Society in London. Marriages and descendants of her brothers and sisters have not been located - one marriage only - her elder sister Jeanne Marie de Grave born 21 Oct 1707, bap 26 Oct 1707 at Glasshouse Street and Leicester Fields Church, apparently married in 1737 at The Artillery Church, Spitalfields, London, to Jean Antoine Megier.