Policy depends on location, but for some places offering your receipt is 100% voluntary. I wouldn't deny showing my receipt at Costco (where it's been standard practice long before self-checkout came around and, though I don't have a copy of the agreement handy, I wouldn't be surprised if it were part of the agreement when you sign up for a club card.) But when I worked at a certain home improvement store, they hired outside security to check receipts. When one of the security guards was ignored by a customer and they asked him again, the customer complained. Subsequently, the security guard got fired. That's how I learned that the policy is "ask once, and let them go if they don't respond the first time." AKA security theater.

My favorite is "Thanks, you too 😊" and just keep walking