I'm growing infuriated by the number of people overseas asking if any purchases made now will reach them. It states it clearly in my shop announcement - like most other people - but they don't read it.
Fortunately, only one person has purchased something from the US and has NEEDED it by Christmas. I told her how much extra it would cost her to guarantee it, and she happily payed it.
I felt bad about having to charge her more, but I remind myself that that money ISN'T going into my own pocket.
If people ask if something will reach them in time, tell them how long it typically takes, and times it by 2 for the holiday rush. Then let them know what the best you can do is: ie, how much extra the faster shipping option will cost, and whether it WILL reach them in time or not. Some people will be happy to pay, others won't respond to your message.
As for people who have already placed their order, tell them the same thing, but also send them a paypal invoice right away for the extra money to help speed the process along. Be sure to mention in the message to them that you have sent them an invoice, and if they would prefer to keep to standard shipping, not to pay it and let you know so that you can post it and cancel the invoice.
It might alarm them to see an invoice in their email, but if you explain in the message, AND on the invoice (and be sure to use the word "optional" in the invoice message, and to give a clear subject line if you are able to change such things), then the panic should dissipate quickly enough.