Not sure how much stuff you need but you could always take the bus to DTD and then walk to crossroads or even Winn Dixie. If it's not too hot and you are up for the walk it is not too bad. We have walked to Crossroads from DTD and the Winn Dixie is a couple more blocks. Or you could just rent the car at the airport for one day and return it at the Dolphin or the Car Center. One day would be cheap and you could get your shopping done and return the car and take one way transfers back. We usually take some food and then buy things like bread and perishables at the store. Yes they cost more but not enough more for the cost of an alternative. We take canned meats, a small cooler with ham and cheese, canned soups, peanut butter, coffee, and snacks. It takes up a small suitcase and it's not the best meals in the world, but for 5-7 days it's fine. I cook every night at home and a easy meal on the go is more than fair at WDW. We eat at counter service once a day but breakfast and a sandwich in the room every day works well for us. We may pay $10.00 MORE per trip for what we buy in the store, that's more than worth it. Good luck planning.
