Couple weeks ago I bought a 16" fan, and I carried a knife with me to the store. The knife wasn't to negotiate the price: Whenever I have to carry a large cardboard box home that I know is too big to fit into a plastic bag, I save the shop some twine by bringing a knife. I then cut a slit into the top that looks like this, |____|, basically three sides of a rectangle, and push that part into the box, making a convenient handle.
Of course this only works with items you can carry with one arm; for heavier stuff, the Move-It Kit looks like it might be a good solution. It's basically an all-cardboard series of add-ons that turns your average cardboard box into a carry-on suitcase. Check it out: