thanks for the responses guys
as I said, I could use the American Kindle Store for over a month, even in Brazil. I bought like 7-10 books and countless samples before it stopped working for me.
1) The US address I used not only is valid (since its my relative's), but is also on my Address Book (I've already bought a few paperback books from Amazon and sent them there).
2) I think the problem for me is all about my physical location. When I try to buy a book on the American Kindle Store, it says:
"XXX, please contact us or change your country to complete your purchase.
You attempted to purchase an item while in a different country than listed on your Amazon account.
Are you traveling outside your country?
Please contact customer service in order to continue purchasing Kindle items.
Did you recently move to a new country?
You can easily update your country for your Amazon account."
You see? I don't think buying gift cards would solve anything either, since the problem seems to be where I'm buying the books from.
I might try to proxy thing, I'm just worried about its security. Does anyone know if it's safe using one?