Nothingness what a pain!!
Did you happen to take up the Tracking option when you bought it?
I bought my cover, which appears to be the same as your cover, last Saturday and I've had 2 emails from Skin Digital since advising me that it was preparing to be packaged and then that it was shipped.
I also got the Tracking order to follow it's progress via Australia Post.
In your situation in the US (?) your package may have been held up in the Christmas rush at the US end, in Customs perhaps, still waiting to be checked?
I'd send Skin Digital an email asking them where it is.
Our postal deliveries won't start here again until Friday this week due to the Christmas Public Hols, so I wouldn't expect to see it till Monday or Wednesday next week at the earliest. (Tuesday New Year's Hol)