As I know, the tax-free allowance is about $50 and it works only when a parcel is handled by Correios (the national post), for other logistical companies (like FedEx) you will pay taxes and customs fees (practically once it cost me +100% to the raw price from Amazon US for a parcel below $50).
[I thought a little later] Maybe you mean for orders from international e-commerce stores? Because paying for taxes and customs fees for goods which you bring by your hands in your luggage during traveling by flight sounds really strange. Of course, I mean the realistic amount, not gray import like bringing dozens of iPhones in bags... For example, as I know, in Russia it was about the equivalent of $10000 in cash.