Milly is driving to Greytown when she sees a self-service stall selling apples. The sign reads, "Pacific Rose apples $5 per bag. Help yourself. Please place the payment in the honesty box". Milly selects a bag and places $5 in the box. The farmer then arrives and tells her the price has gone up. He demands that she pay an extra 50 cents for the bag she has selected.

Required: Referring to legal principles of contract law, can the farmer insist that Milly pay the extra 50 cents?

Answer in essay format and use APA referencing within the answer like an essay.