I've just discovered the other day that the following was not well known:
As a published author, I contribute bookmarks, etc. to different conferences, etc. When the conference is over, there is always a very polite thank-you note acknowledging my contributions. Usually, these are bookmarks for goody bags, but authors send all sorts of stuff to fill these bags. They are taking time and effort to help a conference (okay, it is promotion for the author, too :))
The thank-you note varies. It can be a brief e-mail, or a sweet little note (always great to open), or in the form of a business letter if books are donated. This is something that needs to be tucked into my tax file to show I am using my ad books.
This courtesy is customary, and fuels more donations from authors. Rarely is this courtesy skipped.
If you are having a conference, or promoting a cause, take time to write that little thank-you note to the people who have donated. It is so very important.