Alcohol is very expensive in Iceland and you can only buy it at state run stores called Vínbúðin. The DutyFree at the airport is a lot cheaper. Strong liquor is a lot cheaper while wine and beer are also quite a bit cheaper. We recommend buying some while waiting for your luggage to arrive.
P.S. Do not be fooled by the “beer” you see at local supermarkets. Those are light beers with only about 2% level of alcohol.