I also go to the Costco in Lihue. I only pick up a few items there because the pack sizes are large on most items. I usually stay for two to three weeks so it works a little better than if you are only staying a week.
If you do have any left over unopened food, you can donate it to help some locals in need. There is a box in the lobby of the Pono Kai where you can place these items to be picked up.