The only service in Argentina I know of is OCA-box:
http://www.ocabox.com.ar/
I tried to sign up with them online and resend some letters to myself here, and the package was lost. And visit to their International office on Peru street did not help at all. So, I would not recommend using them. May be somebody else had better experience?
I am using US Global Mail
http://www.usglobalmail.com which is based in Houston, TX. Never had any problems with them. But letters and parcels nobody has to sign for are often getting lost here in Argentina. So if what you send is not a merchandize and there is no way to buy insurance, sending it by Fedex may be essentially your only option. Fedex also clears customs for you and collect custom duties on delivery. With cheaper options like US government postal service, you have to do it yourself at international postal office by Retiro station.
The only problem with Fedex is that it is expensive (46 dollars for Fedex envelope), so if there are people who would like to share a mailbox that would be great. We could resend all the mail in a single package and distribute it here. Or may be just bring it to the next dinner.