This piece of Carl Barks art recently surfaced at auction.
A post for those of you who has already seen "everything". :)

Image courtesy of Hake's.
Handwritten text at top “Hi, Mr. Matetsky-We Deeply Appreciate Your Interest In Us Ducks!” with additional text just below scene “-And So Does Carl Barks”. Handwritten text on bottom half reads “Dear Mr. Matetsky-I Wish To Thank You For Your Generous Valuation Of My Duck Paintings. I Try To Make Each One As Good As My Skills Will Permit. You Certainly Make Such Hard Work Worthwhile. Gratefully Yours, Carl Barks”.

Bob Pistella got another nice letter drawing in his comicartfans gallery. Click HERE to see it.

To round off this post, here's a letter + drawing sent to Jim Lowe in 1974: