Okay - contrary to most people on this post - I HIGHLY recommend using Correo Argentino (and seeing as the posts HERE are from a few years ago, I'm glad to update this post with an experience from 2016!). I have been living here for 6 months, I have sent TWO IMPORTANT packages to the US. I've had no problems. Read on for details...
1. I had to send keys to someone - I used the regular service, without verification because I didn't have the extra time to wait. They arrived within 8 days, no problem. Genial.
2. I had to send all my personal documents to the FBI because I'm still working on my visa paperwork (BY THE WAY, if anyone needs advice on getting fingerprints done here in Argentina, message me). I sent these certified. They gave me the RR number. I verified that the packaged left Argentina on the correo argentino website. THEN (and it seems there's confusion about this below), I used the SAME number (but added the AR at the end of the number) to verify that it was indeed DELIVERED in the UNITED STATES, via the usps website. The only complaint I have is that the packaged took a little over three weeks to arrive (but it LEFT Argentina the day after I sent it, so whose fault is that??? The UNITED STATES or ARGENTINA??? Hmmm...)
Unless I'm sending a diamond bracelet, I'm not interested in spending ridiculous prices via FedEx etc.
Go to the Correo Argentino office nearest you. Take a number like all the Argentines do. Wait your turn while contemplating how organized and civilized the number-system in Argentina is. Then, send your package! Whew. We aren't in the US, but somehow, in 2016 things magically still work.
