I understand why SOT wouldn't want the responsibility of a negative list, and it seems many customers pay some people and ignore others.  What about having an accounts payable contact list?  In my experience in this job and at a prior job in accounts payable, it seems that people get the best response when they're able to contact the same person over and over and over again.  It might be nice to have a list with names, extensions and email addresses of people who can actually get things done.