I suppose you could bring a wagon with you and stow it under the cart while you're shopping, or maybe chain it to the bike rack.
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How about if there was a small fleet of wagons at the dock, and folks could rent 'em like they do at the airport. You could drop a couple of bucks and go shopping wherever you please. When you're done you bring it back to the same place. It might need a card swipe or lojack to keep track of them. It'd be nice if they floated too.
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In the case of Alton Bay if you needed to get to Hannafords from the town docks and you didn't have wheels, perhaps a Hannafords van could pick you up and drop you off. You would pay for the ride to-and-fro at the store, just like another grocery. Not that they have such a thing now, but it's just an idea...
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Call a taxi.