“We’ve all been there: A store cashier asks if you’d like to donate money to the local food bank. Or the PIN pad at the checkout counter prompts you to round up your payment for charity — spare a little change for a worthy cause. Those ’round-up’ campaigns have become ubiquitous in recent years — at grocery chains, gas stations, retail stores and online merchants.” Well, keep opting in. That spare change you donate at checkout is adding up to millions for charities.