Ok well to update -- my husband was trying to buy a ticket EZE - SJC -- we got mine and the kid's with airmiles (we have to travel for my sister's wedding, she decided to boo her wedding date for exactly one month after our child's 2nd birthday, boo on her :S ). Anyway, so he checked flight prices on AA, he is super elite or whatever AA's levels are -- platinum or whatever. The price was showing as 1225. He reserved it and then the next day when he goes to book it comes up as 1446 due to location of billing address. So he calls AA, and he talked to 3 different people, spent over an hour on the phone arguing with them, they were saying that no when he reserved the ticket he had used a US credit card, he said impossible because he didn't reserve with CC, he reserved with his AA member number which clearly states he is in Argentina etc. Anyway, in the end, he argued so long with them that they went into the whole system and changed the price to US$1225 "just for him, just this one time" etc etc. So it seems the price change on the AA site was not the AFIP charge, it was the "screw you, you're booking as a non-US resident charge". If this is true, than he saved himself a bundle since the AFIP charge would have been put as 20% of the 1446.
Re buying tickets in pesos, not sure -- we had no problem, gave the CC number and that was that, there wasn't any talk of AFIP approving sale, just said he can pick up the ticket at the AA office in 24hrs. And he's got a CUIT anyway so that wouldn't have made difference (he's local, earns pesos, and is charged official rate on his card for the purchase + at some point we'll get the 20% tax show up on the bill).
