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From their Facebook event page:
On TUESDAY MARCH 31st, come shop at Whole Foods in Mount Pleasant - whether it be for a cookie, lunch or the whole week's worth of shopping - your purchases will end up helping raise money for The Sustainability Institute. 5% of that day's total net sales will be donated to the Institute and in turn, into our communities. SI staff will be on hand all day long (probably eating), so please come out for a visit and contribute to the 5%!
Of course, you could always just donate straight to them.
The store address is 923 Houston Northcutt Boulevard.