You will have to call and pay the phone ticketing fee, but if you're flying coach, I would really recommend using your AA miles to get flights on LAN. They can mix/match with AA flights if one leg is not available. We did this for 35K last Sept. and the LAN flight down was much more pleasant than the AA return.

Also, if Lima is not your only planned destination, keep in mind that you can get an additional domestic flight on LAN Peru tagged on to each of the int'l flights, with a stopover in Lima on one end or the other, for no cost beyond the departure taxes. There is a thread in the AA forum about which combinations of stopovers and open jaws are allowed.