Update on this post.
I received the spare parts from Florida
. They were "under warranty parts", so the invoice was USD0.00.
Nonetheless, as predicted by Copacabana, the authorities charged me 80% on the shipping costs.
There is really no other way around, I've talked to "despachantes" as suggested in this forum, and I also talked to the Customs themselves (Receita Federal).
The answer from the Custom authorities was kind of funny
: "you are right, no import duties should be charged to an in-transit vessel, but we are sorry, we have never got such a case before, so we don't know how to handle it, there is no process in place".
They really wanted to help, but apparently there is no directive on how to handle such cases, so they have their hands tied.
Generally speaking, in my dealing with authorities in Brazil despite all the bureaucracy, I found officials very honest and agreeable people. I've been always been treated with outmost respect and courtesy.