I order books from amazon all the time. They always open the package but then they send it through. Once DHL called me to get my passport/DNI number because the government required it. But other than that I have never had any problem.
I've had two books sent from Australia - the one that was sent registered post arrived in about three weeks, the one sent in the regular post never arrived.
We've had Christmas presents arrive from the UK up to 6 weeks late. We're still waiting for some to arrive. We had 5 packages all arrive at the same time last week which had all been posted at different times in the UK. As with most things in Argentina which have to pass through some form of officialdom the results are completely erratic.
Like many other things I find it so depressing that it is expected/accepted that this just how things go in this country. Yes, we're aware of Argentina's apathy to the rule of international law, but perhaps it's a basic ideal I have that mail should always make it to the recipient. If the IMF can't get Arge to play nice/fair I suppose the same is to be expected for the UPU...