The real issue is that Xmas is over; no more buying presents. This year was predicted to be the most travel heavy Xmas ever; people still working on getting home are unlikely to be shopping on Amazon. And many have realized that they've spent enough money already.

If you sell the type of item that people tend to buy for themselves, you can often see no drop, or even an increase, in sales after the holiday. But most categories see a sharp drop for the last week of the year; totally normal (although this year may be more significant than others).