Do I need to pay extra to go to the United States ?

I have long distances between the cities I stayed in Canada and the US, so I just rented cars locally and did not cross borders.

Not quite sure what it means, is it from Canada to the United States (returning the car to a different location)?

There are also many cases of additional payment, as well as cases of no payment. Do your homework and check if there are any tolls on the route. The best way is to ask the staff when you pick up the car. Many cars have built-in systems similar to ETC, especially on highways in the eastern United States and on the bridges in San Francisco in the western United States.

I always arrange for the highest insurance coverage when ordering a car domestically, so there's no need to worry about anything afterwards.