This is actually not a "sale" from American airline website, it is a mistake fare on www.lan.com. Lan website forget to include YQ surcharge on the ticket, which will result price much lower than other places.
This "sale" is actually valid on lots of destinations and dates as long as you book on lan.com
Detail about "deal" discussion is here and here
too bad, I cannot take advantage of this offer. Enjoy!
Lan.com is official website of Lan airlines, part of oneworld alliance.