It is definitely about both mail and your luggage. The link in my previous reply specifically mentions luggage, including a whole FAQ. Tax free limit when entering by air is 500 USD.
May I ask where are you from? I am a bit puzzled by your surprise because I do not know a single country that does not have similar laws. The whole EU has analogous rules, for example (and I live in the EU). Those absolutely apply to your luggage, and also mail. The difference is in implementation details such as what qualifies as personal use, what is the tax-free allowance, and what are the fees. But the concept itself is not something odd that only Brazil does.