Don't know about US case law, but UK and NZ, the pricing when selecting is considered an "invitation to treat". When the product is actually brought to checkout an offer is made at the listed price on the checkout, which can then be either accepted or declined.
Was brought up after a business was done for selling switchblade knives that were only on display - as no offer was actually made and accepted they weren't "being sold".
To answer your question more directly, prices listed on advertising and boards are considered "invitation", not an offer and acceptance. It can change until the final offer and acceptance is made - bullshit but legal.