See also the institution of the General Post Office.
The overlays that highlight 17th century London features are approximate and derived from Wenceslaus Hollar’s maps:
Open location in Google Maps: 51.513889, -0.088617
Cumgranissalis • Link
The first Two references be for the Poste House, where the mares and nags be stabled, available for rent to those that be without carriage and 6 , also they be used to send by carrier letters and packages to the the outer reaches of the kingdom.
For a comprehensive description of the development of the Post Office I found a free book on line:
THE HISTORY OF THE BRITISH POST OFFICE by J. C. HEMMEON, Ph.D., published by CAMBRIDGE HARVARD UNIVERSITY in 1912
The first two chapters cover the early years and Pepys' times.
Chart showing the number of references in each month of the diary’s entries.