If you will ship a large shipments, you can call the shipping company directly.Or they will not accept your booking and you will not get a best price.
And most of the shipping companies have their own agents,they won’t do business with others except their agents.
It is better to cooperate with the freight forwarders,because they have a full system of services,they can give you a better price and they also have enough experience to deal with all the problems.