1. Buy alcohol in fríhöfnin before you leave the airport. (Cheapest)
2. Buy alcohol in Vínbuðin. (next cheapest) 3 Buy alcohol in restaurant (buy it when it's dark outside. nobody wants to be robbed when it's bright outside)
Never ever buy water. Get it from the next crane.
Shop in Krónan, Bónus or Netto. don´t shop in 10/11 and Iceland