Actually, SAT scores arrive electronically and automatically get filed in the system. If you have December scores sent, we can be certain that the scores will be in the file when we open the application for the first time. That doesn’t mean you aren’t allowed to send scores after deadline, it just means you have to understand that your file may have been reviewed a few times without those new scores.
I’ve addressed the timing of score delivery and the logistics repeatedly over the last few years on my blog. [Here’s the most recent post](Notes from Peabody: The UVA Application Process: The Timing of SAT Score Reports).