Argentina is a complicated case these days. Their policies to survive the global financial crisis survival involve limiting foreign goods to foster their national industry and at the same time avoid spending foreign currencies. This will not change soon.
In Uruguay at the time there are not limits to importation. "Ships In transit" should not be charged taxes
, but the customs people are not easy to deal with. You have to use an agent ("despachante de aduanas") that charges a big chunk of money
for little work. They handle all the dealing with customs, the good and the bad. All and all, think a minimum of 30% over the cost (product+shipment) in fees
In Montevideo I recommend asking the help of Bermúdez (chandler) at the yacht club at Puerto del Buceo. In Piriápolis there is another helpful chandler (speaks French and English), Martins, a few blocks from the harbor. In some cases might be cheaper buying