Humm the U$50,00 threshold for importing is when you send something by 'mail/fedex/etc'... for example.. if I buy something from Amazon, and the item will be shipped from USA to Brazil, I will pay 100% taxes if the item value exceed U$50,00.
When you take a flight you have U$500,00 limit to 'import' stuff ( to bring home the stuff you bought 'cross border' ).... Above this value you must pay taxes... for example, if you buy something for U$600,00 and you declare it ( you don't try to sneak ) you will pay taxes about the exceeding U$100... so... the final value will be: U$500,00 + ( U$ 100 * 1.5 ) = U$ 650,00 ( 50% taxes on U$100 )
If you try to sneak the stuff, and you are caught, you will pay 100% taxes on the full price... U$600 * 2 = U$ 1200,00...