They mean the same thing, with the second sentence having the added implication that the person is doing it very eagerly and to his advantage. There might also be the difference that one would use the second sentence if he wanted his writing to sound classier and more intellectual. And there might be the further difference that the person in the second sentence might appear to be more intelligent and refined than the person in the first sentence: to simply seize the chance to go shopping might be interpreted as crude, but if you "seize on," that implies eagerness but maybe with a some thought to it.
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