So I’m doing another subject test in January, and I’ve noted it in my applications that I would. If i don’t feel like I did well on that day, will I have to send it over? or, can I call the university and say I’m not planning on sending it over?
If the scores aren’t required for admission, then I don’t think you are required to send them even if you noted them in your common app.
However, I would assume that most schools that take January SAT scores would require you to have the college listed as a score report choice BEFORE taking the test (and therefore before you see your scores). This is so that they get the scores early enough for them to be considered with your application.
Check the college’s website or email admissions to check if there are any specific requirements for sending January scores.