"Wipe" is a bit of a stretch and a bit specific when it comes to animal behavior, but many animals do clean their food or clean their living quarters in a variety of ways.
In addition to the other examples already given, I'll toss Eusocial Insects into the ring. This includes groups like bees and ants that live collectively in colonies. For example, honeybees will clean their colony's comb to keep it free of debris. Leaf cutter ants depend on a specific type of fungus that they cultivate for food, and they spend a lot of effort keeping their farmed good nice and sanitary.
I'm thankful that in all my years of working in retail, I never had a gun pointed at me. Having said that, random people get so bent out of shape about the most ridiculous, inconsequential things and it can be close to impossible to predict who it's going to be or what is going to set them off.
Based on my experiences, if a customer is being indignant even after explaining that I was helping them out, I would apologize and remove the discount or ring the order up differently (if it was in my power to do so). More often than not, they calm down after they get what they want, even if it's to their own detriment. Granted, after calming down, a not insignificant number of these folks will realize their mistake and then they have to figure out a way to save face so they can ask for the discount to be reapplied.