Mana foods in Paia has lots of local and organic groceries. It’s all great stuff, and probably close to where you’re staying, but more pricey than the big supermarkets in Kahului, so you probably don’t want to do all your shopping there.
Safeway is generally more expensive than Costco, but has normal sized packaging so is great when you just want a small amount of something. You should definitely get a Safeway card right away as it’s free and gives significant discounts on some items - overall it works out at about a 10% discount.
Costco in Kahului is members only, and is $50 to join. It’s a wholesale setup which means you can only buy large quantities like three huge jars of pasta source, or a tray of 24 peaches. However the deals can be amazing, like 24 cans of coke for $7, about 30c a can, so it doesn’t take long to make back the $50 membership fee - especially if your accommodation has a fridge big enough to handle the pack sizes. Though if you’re shopping for yourself you would need to be happy eating a lot of the same thing for days on end.
It’s also a good place to go straight after a sail as they always seem to have a few free food samples to tide you over before dinner!
The pizza at Costco is a pretty good deal, about $10 for a huge pizza easily enough for 2 people, maybe 3. It’s also available by the slice if you need a quick feed after windsurfing!