The only remotely logical scenario I can imagine here is that two parcels were opened by customs, the poster and a clock, and they mistakenly swapped shipping info once rewrapping them - but there would have to be a seriously inept customs officer to do this!!  (Or very hungover).    A photo would be a very important place to start!

GenFac Plastics are an Australian Company so perhaps the above scenario is possible, although from their website it doesn't appear that they sell clocks.  . .

I would contact GenFac Plastics and ask them (preceding this with "I have a strange query . . .") if they sent a clock to the US with a Christmas card, or if one of their buyers has complained about an item not arriving (or a poster arriving instead LOL!).   And let your buyer know that you have done this.  At least your buyer will know that you are doing what you can to understand what has happened which can only be in your favour.