Converting a portion of your everyday purchase into a charitable donation

If local businesses can get access to a mobile platform in which a portion of a consumer's purchase at their store becomes a charitable donation and the consumer shares that experience on their social media (kind of like a Foursquare check-in) would you use it?<p>How would you envision the way this social venture work?<p>I was thinking of some sort of QR payment app where the consumer can go to a store hosting a donation campaign to make a purchase and also at the same time donate to a cause of their choice. This would basically be a social media marketing tool for local businesses and a fundraising tool for charities.<p>What do you guys think?