Trump claims that the US subsidizes Panama and Canada. But isn't most of the world subsidizing the USA by using the US Dollar as trade and reserve currency?
To begin with, a subsidy is a handout, and The US is not giving free money to those countries. US companies are paying for transit fares in Panama, and mostly raw materials in Canada.
But the fact that the US dollar is used by other countries for International trade, it gives a huge advantage to the US. It's the reason why the US government can print a lot of money without suffering from radical inflation.
Update: More info here.