This is the first in an infrequent series of things (vegan or not) that I like and recommend.
The RV recommends: Saying "I understand."
I'm not gonna get all pop-psychology here, but it's amazing what saying this phrase can do. It tends to bring out the best in people.
The cashier can't give you a refund because you don't have your receipt? You understand. (And you'd be surprised how often rules can be bent.)
Someone is upset with you? You understand. Watch the situation start to get better from that point.
My UPS man wouldn't leave packages at my door because of the risk of theft. It was a major inconvenience because the UPS pickup location is 30 minutes away, but the next time I saw him in person, I said I understood. Since then, he's left packages on my balcony without being asked to.
It's important to be sincere, and not to say "I understand" in the hopes of the other person giving you what you want.