Amazon doesn't charge extra for Brazil, Mexico or Canada. They are all included as part of the unified North America Account. The $40 a month selling fee you pay covers them all. Sometimes they split that fee between the countries, so you might see a $17 charge in the US and another amount in another country, but if you add them all together (after converting them all to US currency) then you'll see it comes out to the same $40.

There is no such thing as a "USA account". When you sign up for Amazon you are signing up for the North American Unified account which includes 4 countries. You aren't required to sell in those other countries, but they are still included. I would NOT try to "deactivate/delete" Mexico. There are plenty of horror stories in the forums about sellers who did this, just to find out it deleted their entire account (again, because it is all one account).

As a side note - you only get "charged" (the fee gets split between the countries) for these countries if you have traffic coming from these countries to your listings. I don't know what that traffic looks like, but if your fee has been split with Mexico for 3 years, it seems to be there is some interest over there for your product.