If you have just sold something and your despatch is set for 3 days the 24th would be the third day. The last date for second class is only Royal Mails cut off for delivery by Christmas. There will be deliveries afterwards, it is only Christmas not the end of the world.
If you are worried about customer expectations you could always message them and say something like "Thank you for your purchase, please note the Royal Mail last postage date for delivery by Christmas has passed so your item may not arrive until after Christmas".
