Postal Rule

Sir,

May I have your help with this question please?

Thanasis owns an art gallery called Kappis Art. On Monday he place a notice in the window of his gallery that said, “2 copies of a very rare Anderson print – $4000 each.”
Constantinos and Georgia were keen to acquire the prints but thought that the price was too high. They each offered Thanasis $3500 but he refuses to sell the prints at that price, although he does say that he will accept $3800. He also says that, in order to give them time to think about it, he will leave the offer open until midday the following Monday if they each will pay $100 immediately.
Both Constantinos and Georgia agree and hand over their $100s to Thanasis. On the Saturday of the same week Constantinos and Georgia were called away to Athens on business but before they left they each mailed a letter saying that they would buy the print at the price of $3800.
Georgia’s letter arrived at 09:30 on Monday morning but Constantions’s letter was delayed and didn’t arrive until Tuesday. As it happened, Thanasis had already sold both prints during the weekend.

True of False: Because his letter arrived after the deadline of midday on Monday, Constantinos will be unable successfully to claim that he is entitled to an Anderson print from Thanasis (Correct answer: True)

My question Sir, is: doesn’t the postal rule say that once the letter has been sent through the proper channel that the acceptance is effective from the date of postage? If yes, then why is the correct answer ‘True’?

Thank you for the clarification.