Their standard answer would probably be: "we only accept authorized negotiable instruments and yours doesn't qualify", or something like that. The issue is to force them to either accept it or tell why they won't.
When they say cash, check, money order etc, they are in violation of HJR192 and Public Policy 73-10. Also, the refusal to accept payment means that the debt is discharge under UCC 3-603(b). The toughest part is to convince them that they can't get any farther than this.
If you want to hear something interesting, listen to this file, it is only 3 minutes long:
http://www.suijuris.net/seminars/how...-%20sample.mp3