A handwritten note will usually suffice for a heartfelt thank you.
I have an extensive library of very old recipe books, including several "autographs"—original, handwritten, unpublished, personal cookbooks—that date back to the early 1700s.
Equations must be typeset, preferably by Equation Editor in MS Word, not handwritten.
It was in a business-size envelope, just one sheet of paper and handwritten.