You can't compare Cargojet and ACE, they are two different stories.
ACE was a straight wet lease, the same as when they were operating for UPS. The reason why all this has come about is because Cargojet operates a Co-Load network which allows the customer to purchase as much space as they require, and then Cargojet fills the rest of the aircraft up with their own freight.
With the AC contract, it is slightly different where AC has wet leased the aircraft from Cargojet, however, Cargojet has agreed to purchase x amount of space from AC thereby generating more revenue for AC Cargo.
You can't say that Cargojet undercut them - it certainly has passed on a great savings to AC, but it is because of the fact that Cargojet has its own cargo to move during the day, not because they are giving away their services.