A direct smile at a cranky cashier can change their entire day and perspective on the situation. Plus, smiling is a form of bonding—and people are more likely to help others with whom they feel a personal connection.
Simply offering a genuine smile and politely explaining to the cashier that the store doesn't lose money off of coupons could change an entire checkout transaction. Some cashiers are not aware of how coupons work and feel like transaction totals lowered during checkout reduce the income the store makes, thus putting their personal paychecks in limbo.
So the next time you run across a less-than-delightful cashier, remember: a smile really does go a long way!