What are you going to do with that evidence that walking back to the shop won't accomplish? If I'm too far away, I'll throw the donut in the trash. If I'm nearby with nothing better to do, I'll stop back and ask for a refund.

Nothing about that practically requires a receipt. If I'm denied a refund when I go back with an insect-laden donut, I can make enough ruckus in the shop to get my entertainment value's worth. I don't really need the $0.75 anymore than I need the donut...