You've got so much handwriting here you may not need to get it authenticated. Let me take a look at possible value and let you know what I think. A member here might be interested in it.
That's a great letter you have there. I don't know how scarce they are but it is desirable. I just took a quick look at RRAuction archives. The yellow Seinfeld handwritten monologue sold for $349 in Dec. 2009. The 1984 contract brought $625 in Feb. 2008. That's all they had besides a 49 page Seinfeld show handwritten draft script for $8,230 in 2006, but that's a whole different animal.
I see John likes it. He's a dealer but he steps out for stuff he really likes.