So, according to the USPS, as long as orders are shipped out by the 20th of December, they should arrive before Christmas. :) This goes for First-Class (which is what I use to save customers money) and for Priority Mail.
December 19th is the last day to place an order in my Etsy Shop for guaranteed delivery by Christmas. That way I can make sure that all orders go out by the 20th. If you’d like to be even safer, orders placed by December 17th should definitely arrive before Christmas.
Just giving everyone a heads up. I’ll probably reblog this closer to the cut off dates. :)