We don't sell hand made, but we have the same problem this time of the year. I really struggle with responding to international customers who are buying. This is what I tell them:
"We cannot guarantee delivery times on international packages due to Customs and Custom inspections. You might consider quicker shipping by checking "Express Shipping".
On US packages we tell them that because of the holiday season, we cannot guarantee arrival at a specific time, but that they can choose priority or priority express if they want to guarantee over night shipping (except on week-ends). We have been able to send out many over night packages that have arrived on time. We also, in the past year, had one over night package (placed on Friday night) end up not arriving until Monday. There is a part of shipping that is out of our control.
If it is a matter of getting a handmade item to them on time, I think that is up to the shop owner to make sure that either A) it gets done even if I don't have sleep, or B) I let them know ahead of time that I can't get the order filled in time.
Just a take from a different perspective, which is primarily inline with what the rest of you are stating!
Merry Christmas!
Gayla