Today I got an invoice from a major, world-wide known car rental company. It seems that I have used a toll express highway without paying and I also got a speeding ticket during my vacation in Canada. I don't want to discuss the ethics here, but is there any drawback if I don't pay the bill? Would I get into trouble when trying to rent a car with the same rental company?
If you rented the car with a credit card, you will have signed something that says they can charge these things to your credit card and it's not disputable. The only speed-camera ticket I ever got was in a rental car (and they tacked on a handling fee to boot.) It took weeks to come through, but it came as a charge on my credit card.
If you had driven your own car on the 407 and then not paid the toll, there would be consequences for the car. (At one point, you couldn't renew your license plates if you had outstanding tolls against that plate.) Presumably the car people have paid the toll so that their car can still function. You now owe them the money.
I am completely sure you've signed something that holds you liable for these charges. Since you're describing an invoice, you must have rented the car with cash or cheque. Whether to pay or not now is no different than if you had ordered something and then not paid for it once it was delivered. The country as a whole is not in the habit of enforcing these sorts of debts, so you probably won't be nabbed at the border, but you can be certain that the rental company will keep track of you. If they hand the debt to a collection company, it's possible the collector would find it worthwhile to harass you from a distance. They can be pretty nasty.
To me it's simple. You signed that you would pay such things. Pay it. If it's a lot of money and you can't afford it right now, contact them and tell them so, and arrange to pay it over time. But pay it. And next time stay off the toll highways (they're clearly signed) if it's not your intention to pay the toll.