Arrived Late Last Wednesday for 48 Hours about 11:50 PM flight was delayed and to my amazement the Duty Free Shop in the baggage area was STILL OPEN! I asked the two guys there and they explained unless its going to be rediculously late that they remain open in the baggage pickup area until all the flights have landed for the day..... So regardless of your time Duty free in the baggage pickup should be open.
SIDE NOTE, Both guys were clueless as to if they would be open during the 4th & 5th during the dry time for Elections. They just gave me the Yo no Se response. It may have something to do with being "INTERNATIONAL" duty free, but they did not know the answer so at leat until midnight on the night of the 3rd you should be OK to purchase liquor at the airport.
Hope that helps.