BiCentennial Commemorative Issue Stamp Cover with Blooper!

I have an American Bi-Centennial Commemorative Issue stamp cover from the International Stamp Collectors Society with a couple of typo issues and I was wondering if anyone knew whether or not is was unique/valuable or not.

It features an original signature of Helene D'Pree, designer of the pictorial cancellation and states, "Replica of the exclusive Bicentennial/Centennial pictorial cancellation availabe (should be spelled available) only at the U.S./Stamp Expo Post Office during the 3-day event. To postmark the new 13-cent Telephone Centennial commemorative stamp." (The last sentence should actually be part of the first sentence and not separated with a period.) Included are an Alexander Graham Bell/Telephone Centennial 13 cent stamp and postmark on March 19, 1976 from the Stamp Expo Station.

Thanks so much for your time!