I shop by bicycle. I find I can get enough groceries to last at least half a week into the rear panniers of my bicycle, and if I need to buy more, I can always load my front panniers. The picture to the right tells the story: all the food and drink on the table went into the panniers. I didn't have to pack tightly to fit everything in. The food I buy in an ordinary shopping trip, the kind many people make in a car, will generally fit comfortably enough in one or two panniers.
When I buy larger items, such as toilet paper, I generally use a bungee cord to strap the object to the rear deck of my bike, as the picture to the left illustrates. Usually, a single length of bungee cord suffices to secure a quite large item to the rear of the bike.
I find that I can ride quite comfortably carrying heavy load on the bike. The goods I bought in the shopping trip pictured above have mass of at least ten kilos (weighing twenty-two pounds), more than I would ever want to carry on foot. Riding, I didn't even notice I had them on. In many ways, a bicycle shows its advantages best when carrying heavy loads.