Assuming the online-pickup price is same as the in-store price, you can reserve for pickup. Or even if the airline itself has duty free

https://www.ishopchangi.com/en/categ...ne-and-spirits


https://london-gatwick.worlddutyfree.com/.

You need to research any limitations (eg if transiting somewhere else, does transit allow security sealed beverages), and the duty free liquor limit when entering Singapore. And technically the duty free is supposed to be for personal consumption, not family/friends