In a case like that the customer didn't see the value in your markup. He DID see that you were sending in the same carrier on a regular basis, and they saw an opportunity to shave I'm guessing 10 to 20% off of their freight bill.
The carrier really can't refuse the shipper...The shipper is ultimately the customer, and the shipper could have said fine, we'll find another carrier, and then he as well as you would be out of that business.
Sometimes we need to remember that not every transaction requires a broker in the middle of it, and when that's the case we need to let it run its natural course i.e. letting the carrier and the shipper work together directly. This is why we as brokers need to focus on freight that can justify our margin..if all we're doing is dispatching the same carrier to the same shipper then it won't be long before someone, the carrier or the shipper, sees the value in cutting the broker out of the transaction.