I guess it depends on how often you go, and/or what you're picking up. You won't get multiple cases of drinks, huge packs of TP and PT, big boxes of product, etc.
I go once a week on my normal shopping trip to Costco (same diff), because it's quite close to me, along with the regular grocery store. Today I picked up a cooler full of fridge and freezer stuff, plus a few other necessities. The grocery store added several bags of other things. Sure, I had to put the seats down to fit the large plastic cooler due to the shape of the trunk (smaller at the top), but with them down there's a bunch of room.
But no, you're not going to supply a small restaurant with the cargo capacity of the 500. ;-)