Mr. A managed a self-service pharmacy shop where any customer selected items as he/she wished to buy and placed them in a basket. Mr. A also placed a notice that drugs may only be purchased under the supervision of a registered pharmacist stationed at the pharmacy during the daytime. The payment was to be made at the exit where a cashier was stationed and in every purchase involving drugs, the said pharmacist supervised the transaction and was authorized to prevent a sale. In this situation, assess at what stage a contract is said to be made, why? Substantiate your answer with case law.