Not sure in the US but in the UK, it's called the Right to Treat.
So the shopkeeper displays the goods with prices. The customer browses the products. Now if it was up to the shopkeeper to determine you accepted to buy the item, then he could say you bought it because you touched it. So the law reverses the sake. So the customer decides what to buy and in effect, takes them to the till and is saying, "I wish to buy these". Usually the cashier accepts the contract and rings them through the till.
So some points -
1) As a customer, you could say, "The ticket price sates $10, I offer $8". It's up to the shopkeeper to say, yes, no, or $9 etc..
2) When the shopkeeper is presented with the offer to purchase them by the customer, he could decline and place them back on sale, no excuse needed
3) The item is normally $19.99 but a wrong ticket for $9.99 was on display, "You have to sell me it for $9.99". "No sorry I don't, I don't accept the sake, there's been an error on the price, I am removing all from sale until the problem has been investigated". Hopefully the shop has a price checking procedure in place or trading standards could get upset. If the shop does sell it at a lower price, normally due to goodwill, but they're not legally obliged to do so.
So going back to the thread reading the above, if the customer eats the sandwich and goes to the till, what happens if the shopkeeper decides not to sell it? So technically, it's theft, wait until after the purchase is my advice, but this is UK law.