How many people are in the household and how often do you go grocery shopping?
If it's just yourself and you go weekly or more often, you could probably get by just fine a backpack.
With 2-3 people I'd go with a rear rack. It's difficult to see with the pictures in the amazon listing - does it have eyelets to mount a rear rack? That + panniers/milk crates/garbage cans/whatever else you can find & attach works very well. A rack only costs ~$25 on amazon.
With 4+ people you might want to get a carrier or trailer of some sort. You can generally find these pretty cheap on craigslist & you don't need anything fancy