A price tag is, conventionally, regarded as an "invitation to treat". It's not legally binding as an offer. The legal fiction goes that you take your goods to

A price tag is, conventionally, regarded as an "invitation to treat". It's not legally binding as an offer. The legal fiction goes that you take your goods to the checkout, and the offer occurs when the checkout clerk says "that'll be $19.95", and the acceptance when you offer payment. Alternatively, you could take the goods to the checkout and say "I know it says $19.95, but this is shopworn and I'll only pay $15 for it" - which is then the offer - and the acceptance occurs when the manager agrees to that price.