If this information isn't in your policies or every single listing description, put it there now.
If people still buy at the last minute even once that's there, there's nothing you can do. A surprising number of people buy last-minute and expect a miracle to occur. I recently had such a situation - I had a commission come in in the middle of April. I made the piece, created the custom listing, knowing she needed it by the middle of May - fine, I knew it would be there in time. But she didn't buy it until only a few days before that. I did what I could, offered faster shipping but told them it still wouldn't arrive in time. Fortunately she was understanding, and it didn't arrive in time as I told her, but she was still pleased with it.
Christmas is a nightmare, too. I think everyone should have info like this in easy reach, especially during the holidays, as well as with shipping deadlines.