Non payment of work done

We supp**** services to Saxum insurance on the 23 Feb 2016 and after numerious phone calls and emails to them they still manage to think out a new excuse and have failed to pay the invoice GD14000. We have spoken to Lu-Anne, Elliot, Birnam, Thabogelo, Charlene and Sade. We leave messages asking for call-backs but to no avail. We contacted their client Lauren, which is the owner of the vehicle, and even their own client has no influence on them getting the invoice paid. The windscreen was changed on time and in good service to their client but now we have to waste our resources to collect the outstanding monies. I wonder what will the situation be like when their client has a real car accident claim? Will it be dealt with in the same manner? We have phoned head office *** to escalate the matter but they refer us back to the specific office with the number ***.