Next time at walmart be counter-active by asking the cashier for some ID. Demand verification of their employment before you proceed to show them your own personal ID, and explain it's because you don't show your ID to nobody, because you're a very important person. Grab a brochure, and start asking about it. Point at the pictures until you've rearranged their mind. Then demand they conclude your purchase, and state you've already shown them your ID. Afterward disclaim you're pretty quick; it's hard for people to keep up with you, and you can't work with stupid people.