On 1st February, Arthur advertises his computer for sale in the local paper, giving information on the computer and contact details. Ben reads the advertisement, and on 2nd February, posts an offer to buy the computer for $2000. This letter reaches Arthur on the 4th February.

On the 5th February, Arthur sends a letter to Ben stating: "Please advise if payment is by cash or cheque". In response the next day, Ben posts a letter to Arthur stating: "My price is cash on delivery." This letter reaches Arthur on 8th February.

On the 10th February at 2pm, Arthur sends a letter to Ben stating: "I agree to your offer to buy my computer as specified for $2000 cash on delivery." This letter reaches Ben at 3pm on the 11th February. However, on the 9th February, there is a change of plans by Ben. As a result, Ben sends a letter to Arthur at 1pm on the 9th February seeking to cancel the earlier offer and stating that he was now only willing to purchase the computer for $1800 cash on delivery. This letter is received by Arthur at 4pm on the 10th February.

Is there a valid simple contract between Ben and Arthur for the computer? Justify the response citing case authority whenever relevant. When answering this question, fully consider the relevant Common Law principles, rules and propositions in the Law of Contract. Do not consider any matters that might arise in the circumstances under other relevant legislation.