Dear Customer (name blocked for privacy),
Article 2 of the UCC, Sale of Goods, does NOT cover the pricing of goods (other than how the Statute of Frauds applies to it). Article 2's main concerns are contracts between merchants and comments made to consumers when buying goods, warranties, etc.
Consumer pricing laws mainly do not exist, and the difference in price on an item vs. at the register will likely fall under contract law and can, depending on the facts take one of two turns:
The store has a policy posted about what it will do if an item rings up differently than it is marked. If it does have such a policy then, under consumer protection laws which require honesty in information given to consumers and the UCC which require merchants to honor thier sales polices, the store will need to do what its policy says it will do; OR
If there is no store policy posted, then the store may fall on the old "its not a contract its an offer to negotiate" and the price is what we say it is as we have no contract (common law)
The UCC (even though its statutory), how other consumer laws regarding retail sales and advertising apply in PA will be a matter of precedent most likely, instead of written laws. If you can pull up PA cases through Lexis or WestLaw then you should try to find precedents that way. You may also want to review the consumer information available through the PA Attorney General's Consumer Protection office here to see if they have materials on this topic.
PLEASE NOTE: Responses here are for information/education only, NOT legal advice and do not form attorney-client relationship! Only licensed attorneys you hire in your state can provide legal advice.