With Christmas fast approaching the mail carriers will experience heavier than normal volumes which will extend delivery times.
As a guide, I've noted recommended latest purchase dates to have a chance of delivery before December 25th 2017. These are recommended dates, not guaranteed dates. Not unusually, I've seen US customs delay progress of tracked items for up to a week.
I've included 2-3 days processing time, but this is based only on reasonable endeavors.
Please remember that the postal backlog will extend some time into January - the third week in January 2018 has been suggested to me as when delivery times finally get back to normal.
Thanks in advance for your patience, and Season's greetings to all!