For cheaper shopping, to resupply, buy the clothing that one didn’t bring or that was in the lost baggage, purchase food items, etc. COSTCO is between KEF and Reykjavik.
Costco Wholesale Vorhuis
Kauptún 3, 210 Gardabaer, Iceland
South of Reykjavik, if you’re renting a car at KEF ideal to stop en route to Reykjavik. Or starting out southbound on the Ring Road.