Yes, you can shop online and collect on arrival:
http://www.dfs.com/en/airport-singap...op-and-collect

Alternatively, there are DFS outlets right next to baggage claim belts, so you can even pick up your booze on the spot. The selection is limited, but they'll have the basics like JW, plus they can "order" stuff from the larger shops in the transit area if you're not in a hurry.