Send them a statement Certified Mail.  this proves they recieved your statment.  Any future correspondance also send certified mail.  If/when the time comes you need to take legal action it will prove you provided them with a bill and that they recieved it.

Edit:  This is a perfect example of why I do not sell hay or straw anymore.  Not sure why but for some reason people think there's no need to pay for these items.  Bewildering to me......