If your combined weight is under 5,000 kg. (11,023 lbs.) contact them and they should reduce the charge to a light vehicle rate.
Due to the outrageous toll charges I only use the 407 once or twice a year, usually towing my TT, and I do not have a transponder. The cameras do not and cannot calculate the weight of the vehicle and only get the picture of the trailer plate. On occasion I have been charged the heavy vehicle fee. My combined weight is under 5,000 kg. so whenever that happens I call them and they reduce the charge to the light vehicle rate. I have never had an issue with getting the reduction.