The U.S. Postal Service’s Historian has produced these data compilations:
- Rates for Domestic Letters, 1863-2009 (pdf) (rtf)
- Rates for Domestic Letters, 1792-1863 (pdf) (rtf)
- Rates for Stamped Cards and Postcards, 1873-2009 (pdf) (rtf)
- Pieces of Mail Handled, Number of Post Offices, Income, and Expenses, 1789 to 2008 (pdf) (htm)
- Number of Employees, 1926 to 2008 (pdf) (rtf)
- Delivery Points, 1990 to 2008, in Millions (pdf) (htm)
Source: U.S. Postal Service, Historian, Postage Rates and Historical Statistics.