Amazon posted notices several months ago that they were adding Brazil to the North American Unified Account. You have access to it, but that doesn't mean you have to sell there. You will only "automatically" start selling in Brazil if you have your account to automatically build international listings.

Something to remember about vacation mode (mentioned by others) is that it only effects your FBM listings. If you are selling through FBA then vacation mode will do absolutely nothing.

When it comes to FBA there are two ways that a customer from Brazil (or other countries) can purchase your US product... They can go to Amazon.com and purchase directly from the US and have it shipped, or they can purchase from their country website through the Remote Fulfillment program. If they are going through the US site, you can change that in your settings. If they are going through remote fulfillment, you can go to this page in seller central to adjust the settings:

https://sellercentral.amazon.com/rafn/inventory/home

The nice thing about remote fulfillment is that is costs you absolutely nothing (the customer pays the international shipping/duties) and gives you an easy way to sell products in additional marketplaces. You will have to login to the country specific accounts and update listings, prices, monitor account issues, etc. (its essentially like having a completely separate selling account) but if it isn't going to cost you anything extra, why not use it? We use it and we typically see sales amounting to about 10% of our US sales (without ever having to go through the legal hassles of shipping/selling product internationally!)