Yes, there is mail service, but you have to go to one of the post offices to send a letter. Don't use a mailbox which are no longer serviced. I recently received an apostille mailed from the US Embassy in Buenos Aires in ONE DAY -- Palermo to Balvanera. The postage was 5.50 (two pages in letter envelope). It took a month for the apostille to come from Authentications Office in DC to the US Embassy. And they made a mistake in the street address. At least the postal code helped in the delivery process.
Additional postage information: fast service from Argentina to USA is 49 pesos (4 page letter); normal service for the same is 29 pesos. Rates I paid on October 3, 2012.