Sorry it took me so long to respond on this topic - The note wasn't showing on my other computer.
Anyways, while Sensei is correct about the kanji for autograph, what is commonly more used is the katakana for "sign" ( サイン ) . Occasionally, the katakana for "illustrate" ( イラスト ) will also be present. In the case of your letter, both words are present. However, just because the katakana characters for "sign" are present in an auction does not mean that it is a real signature, or the item solely contains a signature and not an original drawing. This is one reason why it can be a pain to try to find these things on YJ.
Huh..They also use katakana to spell out "creator" too, which, I find interesting because most of the time they're just referred to as 'sensei'. Sorry, just the ramblings of a total nerd
These letters are typically sent out with contest winnings. Yours says that it was for the Animedia July Contest.
I'd love to see the shikishi. I love looking at these things.