My dh and I were at WDW this past January. The cost of a fridge was $10.60/day (incl. tax).
We are car-less at WDW. We took Tiffany Towncar to and from the airport. They (and I think all the car services have this, not just Tiffany) have a 30-minute grocery stop, which I believe you have to request when booking the car. They stopped at the Kash "N" Karry (sp?) which was a huge store (okay, all supermarkets look huge to me, I live in NYC) with a good selection of food and not-bad prices. I was surprised that they stocked the soy milk we like, although other health-food-store-type items were absent. A good idea is to go with a grocery list (I wrote ours on the plane) so that the 30 minutes doesn't turn into an hour.
On the sage advice of people at this and other WDW boards, at the K-N-K we bought a styrofoam cooler, which if I recall correctly, cost $1.59. The coolers are stored in an overhead area, so you may have to ask for assistance to reach them...or even to locate them, and they come in various sizes, so you can get a small-ish or a large-ish one. Even the large ones were quite cheap. We also purchased cereal, bowls, plastic utensils, bread, pb, and fruit at the K-N-K.
We kept the cooler in the room, filled up zip-lock bags with ice (free at all the WDW resorts), changed the bags in the morning and evening, and kept all our stuff suitably refrigerated for the entire stay. Since we were there 10 days, this was a considerable savings over renting a fridge. It is a bit of work, though.
We tried to purchase everything we'd need for the entire trip at the grocery stop, however, we failed in this regard and ended up purchasing more stuff at steep prices inside WDW, although it was probably still cheaper than hiring a taxi would have been.
Re: using netgrocer. I've heard that the resorts no longer want to hold packages for arriving guests and that there may be a storage fee charged if you do have a package sent ahead. I don't have the details on this, but they're no doubt around somewhere on this board.
Not having to pay restaurant prices for breakfast is a truly huge savings at WDW, as is carrying around your own snacks. I would also recommend buying a case of bottled water on your grocery stop unless you don't mind the taste of tap water in Orlando, since bottled water at the parks is pretty darn pricey. We'd taken a small Brita bottle with us, but its filtration system was inadequate to the task. BTW, an aside on water: in the sit-down restaurants at WDW, the water they serve at the tables is filtered, so there's no need to order bottled water. I believe that even at the counter-service restaurants, if you request a glass of water (non-bottled), that that water too is filtered, although I may be wrong about this. Also, you may not mind the taste of the water in Orlando--it's an individual thing.
Enough! Have fun! Enjoy your trip!
best -- miffy --