duncanvr,
your
ebay shop contains many interesting letter.
And time permitting I'd like to add some comments, not necessarily corrections.
I post here but maybe a new topic could be created, I don't know.
http://www.ebay.co.uk/itm/1822-WRAP...2015849026961822 WRAPPER LIEUTENANT ROCH TO LONDON MARINE AGENSTS SOUTHAMPTON SHIP LETTER

>1/5d RATE CORRECT RATE TO BE COMING FROM FRANCE BUT NO SIGN OF ORIGIN.
The charge of 1/5 ( 17d ) was calculated as:
- 8d, incoming ship letter
- 9d, inland rate Southampton - London
The main reason you will find the name of the city inside the ship letter postmark was to allow the postmaster to calculate the inland rate.