will help you run searches to find out what the rates were.

14 May 2007
12 May 2008
11 May 2009
17 Apr 2011
22 Jan 2012
27 Jan 2013
26 Jan 2014
31 May 2015
17 Jan 2016
10 April 2016 (decrease; the expiration of the 2014 "exigent surcharge" permission
                            granted by the Postal Regulatory Commission.)

In the early 2000's, parcel prices (Priority, parcel post aka standard post, aka retail ground) often changed in May, and letter (1st class) prices, in January.

I could retrieve historical data from the post office back 6 years to 2009 by running this search in 2016:

                Price List DMMArchive retail notice
"Price List" is not regulations and forms
"site: " forces most search engines to perform a search of content only within the pages of the named site (Web page URL).
""  Postal Explorer, United States Postal Service, commercial
    The Postal Explorer of the USPS is their repository of documentation.  It lacks an over-arching search facility. 
"DMMArchive" - Domestic Mail Manual.  Recent churns of the DMM as rates change (2009-2016) have been archived under the run-on phrase "DMMArchive", but variations on this search term might find older data that was thrown onto some server with a different file name. 
"retail" "notice" - Retail or "single piece" means us at the counter.  One can mail 50 lbs at a time in an "M-bag" or pull up a semi-trailer to a USPS loading dock.  The documentation available to give these services away to corporations (and libraries and non-profits and poultry farmer chickens) is vast, and is offered as "comma-separated-value CSV" files, spread sheet files, pdf forms, html, etc.  Some of the more  human-unfriendly stuff is intended to be loaded into proprietary software that runs in companies like, Indica, Pitney-Bowes.  Search term "retail" attempts to exclude it, and "notice" attempts to emphasize the retrieval of documents that state what the rates and rate changes are. 

For date-specific searches, try writing the date American style, April 17, 2011.

(Most of the world uses little-medium-big, day-month-year.  Seems more logical.
Most of the world uses the metric system for the same reason: the metric system is more logical than ours.)

Found a good history chart?  Let me know.  jerry-va. I'm at speakeasy dot net.