I would check the terms of your contract with them, then quote it, stating you are tired of being patient, and your creditors are not patient. You performaed the work, this is unacceptable, and that you are prepared to take further action, if in fact you are. But that is me. So far, though I have had some close calls, I have been paid for every shop I have done.
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“Lying in bed would be an altogether perfect and supreme experience if only one had a colored pencil long enough to draw on the ceiling."
~Gilbert K. Chesterton