I forgot to send a test score?

When I sent my SAT scores to the UC’s, I did not see that it also had an option for sending future scores when available. Now, my December scores are in and are a good improvement from my last test sittings. Can I still send them in? Should I? If I can send them in, do I still use the send all scores option? I’ve heard that UC’s don’t allow score choice, but they’ve already received my last scores?

Yes, send immediately via regular delivery (they do not want rush). Do not exercise score choice. There is no harm in resending other scores. Also, and I assume you already know this, you need send the scores to only one of the UCs to which you have applied; all others will get them from the one.