Hi Betsy,
At the 400+ Museums and/or cultural institutions from smallest to largest, I have seen where the store and/or dining (concessions) 'round up' purchases and donate the difference to the museum or a cause of the museum. This includes self-operated and outsourced too. A $10.10 purchase, with the customer's okay, is rounded up to $11 for example, with $.90 going to the Museum. There is signage, of course, so the visitor, member, etc. knows about it and where the $$ go.
I have been to Putnam, long, long ago, and recall it (and the snow that day) well!
Hope his helps.
Art
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Art Manask
Founder/Principal
Art Manask Consulting
artmanask@gmail.com818 358-3588
www.artmanaskconsulting.com------------------------------