I've checked some sites myself Scotty and found some info. Try the Mavs site for some Mail-in info.
The ones I checked recommended not sending in anything to get signed in case it's mis-placed and they can't guarantee it will be returned, or even signed for that matter.
Just checked for some info for ya: http://www.nba.com/mavericks/news/how_to_get_autogaphs.html
You have to be at a home game basically for the above.
Fan Mail -
Fan mail can be sent to:
Coach’s or Player’s Name
c/o Dallas Mavericks
2500 Victory Ave.
Dallas, TX 75219
Found this too! lol:
National Anthem
If you are interested in singing the National Anthem, please send in a CD of just the National Anthem to the Mavericks office, Attn: Game Ops, 2909 Taylor Street, Dallas, TX 75226.
and all you have to do is send in a CD of the national anthem, lol. Doesn't say anything about you having to sing it on the CD