If you can find out who the client is, (is should the management company listed on your shopper instructions form), I would contact them directly. Let them know the situation, and try and find out if they really refused to pay Ellis. Right now, all you have to go on is Ellis's word. Get the facts documented, who you spoke with, when, and all pertinent details. Present your results to Ellis, in writing, copying your information to the Mystery Shopper's Association, Mystery Shoppers Newsletter, and if you live close to Ellis corporate in Texas, to the press (editor) in that area. The Board of Consumer Affairs, or the office of your local state representative may also be helpful. If you're going the political route, DO IT NOW, before the election, especially if your congressman/woman if up for re-election.
I bet you get your check.