I have found in the past that if all else fails, first write a letter to the shipper explaining all details and they are now liable for the freight charges. Courier a copy of the letter to the broker and tell them it is going to the shipper in 24hrs unless you get paid. This usually get's their attention and prompts a return call in a hurry. If you still dont get a response from the broker you probably never will. Then bill the shipper and include a copy of your letter with a p.o.d from the courier which proves that you made every effort to collect.