I am waiting to see how past experience applies here. In Thailand I found that the postal service there co-ordinated with the postal service of the US, under some International Postal accord. Their Customs agency was more accountable and built into Thai Post. I still had to go pick up and pay duty and tax at a center but the packages seemed secure. But you were dealing with the Postal agency not Customs directly. By shipping in other ways you then came into direct contact with Customs and they could steal, an agent could levy a 'surcharge', there were 'helpers' around that would negotiate your Duty for you with their agents. Corrupt and blatantly opportunistic.
I also had a shipment arrive via Fedex, and the Fedex agents were clearly in the pocket of Customs trying to collect exorbitant fees. As my Fedex account was based at the US I was able to appeal to Fedex at the US with the matter and they, seeing the issues, dropped all the fees.
I hope you were able to take your theft to Fedex in a similar way, out of the hands of Argentina Fedex.