You can find books on Historic stamps of America 1978 Postal commemorative society on en.allexperts.com/q/Stamps-Philately-1610/2009/2/books-stamps-great-grandfather.htm
There was no Postal Commemorative Society in 1786
You can find 1978 Commemorative stamps of America on about.usps.com/who-we-are/postal-history/stamps-postcards.htm
The Postal Commemorative Society's official website is http://www.pcscoins.com/ As a subsidiary of MBI (just as Danbury Mint and Easton Press), information about PCS can also be found at http://www.mbi-inc.com/
250-670 dollars
this cover bears the 8cent Lyndon b. Johnson stamp of 1973 and the 12cent freedom of conscience stamp of 1980.u.s mint presidential medal 24kt gold electroplated by the postal commemorative society.
Coins such as these need to be seen before an estimate of value can be made. I suggest you take them to a coin shop and have them appraised.
You will have to look at an FDC catalog. Not all of them identify the cachet or source of the FDC, but it will give you some idea of the value. And you can always contact the Postal Commemorative Society. Most libraries will have a copy of Scott's Postage Stamp Catalog of the World. First of all, you have to identify the specific stamp or card. That in itself can be a daunting task, but most of the catalogs are well illustrated with lots of information. Once you know what item you have, the catalog will provide a set of prices that can be used to estimate value.
The ones purchased from the US Postal Service include the stamps.
In 2001, the USPS issued its only Thanksgiving stamp.
The postal rule is a rule of contract law and a historic ruling. It says that acceptance of an offer takes effect when a letter is handed to a postal worker or dropped in a post box.
A commemorative stamp is issued to commemorate a special event or historic milestone. The US issues such items to mark anniversaries, such as statehood, founding of organizations, and other culturally significant items. They are usually available for a short period of time, typically several months and are printed in limited quantities.
It's value depends on what coins are in the 'collection' and the grades (condition) of them. It needs to be seen. Take it to a coin dealer for a value.