We've ordered things like cereals, granola bars, etc., mostly through their "Subscribe & Save" option. They send an email before a new shipment, at which time we double check the price. The price can really fluctuate, and often items are much more than what our local grocery store charges, so as others have said, make sure you check the cost.
We didn't renew our Prime membership this year, but the Subscribe & Save items always ship free.