I used Amazon on a weekly basis in Mexico and Colombia, but haven't touched them once in the six months I've been here in BA. Way too cost prohibitive here IMO. For example, I recently looked at some specialized shipping tape and the price on Amazon for the tape was $16.99 for six rolls, but the price to ship the tape to Argentina is $64.02. I think books are a bit cheaper shipping wise, but not by much. For example, I just looked up a $25 Spanish picture dictionary and the Amazon price to ship it here is $40, and that's just for one book.
I ended up going with Grabr instead. You go to Grabr's website, paste the Amazon link, and create an "order" indicating how long you're willing to wait for the item (anywhere from two weeks to two months). Someone who is flying to Argentina soon will make you an offer, and if you accept, you pay the money in USD and it's held by Grabr in escrow until you receive the item. The traveler will throw your item(s) in their carry-on and you meet at a safe place to collect your order. Sounds complicated, but it's not.
In my case, going through Amazon would have been $81 for six rolls of tape. Instead, by going through Grabr I only paid $31.89 ($16.99 for the tape, plus $1.19 sales tax, $9.00 traveler reward/tip, and $4.71 fees to Grabr). I placed my order on January 13th, the offer was accepted by a traveler within 24 hours, and the traveler arrived in BA on January 24th, so a grand total of 11 days from start to finish. I met the traveler in his hotel lobby, pushed a button in the Grabr app to confirm I received the item, and that was it. Easy peasy and saved $50 in the process.