Can we defer using cards after seeing them in a lunchbox?

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Sometimes I'm not in need of food and get +50 Food card from a lunchbox which is a waste (or similar case for other resources). After we 'open' a lunchbox, do the resources have to be immediately used or can we hoard them for later use?
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Is it lunch time?! Open Up The Pokemon Sun and Moon Lunchbox and see what we packed!
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