GST/HST can be a minefield where carrier's, brokers, and customers are concerned. One which I am surprised the fed's haven't looked into a little further, because it would be a huge cash cow for them.
Strictly speaking, a load broker is a service provider, not a carrier, and should bill their Canadian based customers GST for all the freight they move on behalf of that customer.
The relationship between brokers and carriers has always been that of "interlining" which is GST exempt. The definition of interlining is one carrier working for another carrier, and even at that, the originating carrier needs to supply a bill of lading in their name for the load to qualify as an interline load.
Technically speaking, and unless someone can provide evidence (rule) to the contrary, a load broker with a basement office and a telephone, without a CVOR or actual equipment, facilitating a move for Canada Bread (just an example) from Mississauga to Los Angeles, would need to bill Canada Bread for the GST on the load broker's profit portion, if the load broker used an escrow account. If the load broker did not use an escrow account, then the load broker would have to bill GST for the entire amount, as there would be no simple, clear cut, method of determining the load broker's profit portion.
Unless I am wrong in my understanding of the GST regulations, a Revenue Canada investigation on the issue, and subsequent enforcement, would pretty much gut the load broker community in this country. It would add an extra layer of administration on the customer's part to claim back that GST, and it would lay bare exactly how much load brokers upcharge the transportation service.