I actually saw that there was a mileage charge when I was trying to use the Bombardier code for a one way out of JFK so didnt use the code. However if you are trying to use the Bombardier code for a one way rental from a non airport location then there wont be any mileage charge.

Check the mileage field before finalizing your rental. If its blank then there will most likely be a mileage charge. I presume for Bombardier its 0.27 per mile. I specifically watch for 'Unlimited' against that field in the rental booking form.