Imagine picking up up a pint of ice cream for $5 and when you get to the register it's $6. This is going to be a nightmare for everyone who works there.
If you catch them raising prices on you in the store, complain. Get a manager. Make a scene. Show them time-stamped photos of prices: "I put a $5 item in my cart and that's how much I intend to pay for it." Do it every day. Give them 1 star reviews.
Consumer backlash is how we stop this. It's the only way. Especially with companies like Walmart that have established themselves as the only option in rural communities. It's not "being a Karen" if you're making legitimate complaints and being exploited.