The buyer would know that due to the timing of the order that there's no way you can despatch in time for next day delivery. Maybe message them along the lines of "thanks for your order, which I received at [time], I will despatch [details]" and perhaps even "I will message you as soon as it's shipped". If it's a new buyer, hopefully they'll appreciate the extra service and become a regular buyer.
Many of my sellers, including still those who sell regularly to me, message me a quick "thanks, will post tomorrow / next day/ after the weekend" and/or "just posted". I appreciate such messages but they shouldn't feel obliged to put in the time and effort to do so, but it's a good way to pre-empt complaints about EDDs caused by eBay's strange & ridiculous sense of time.
eBay expects sellers to have time machines - just nip back to before the order was even placed, and despatch then?!?
