So many of the US based brokers move freight by just saying pick up here, deliver there. The BOL is issued by the shipper and all they do is track it. When customs and custom brokers come into play, many US brokers have no idea and think it all works the same as shipments within the US.
Because of their lack of knowledge and or caring, the carrier is delayed and additional costs rack up.
I've always said, when taking loads from a US broker, ask about the customs broker and customs paperwork before you even agree to the rate/load. This way, it is front and center and not an afterthought.