<p>Lisa,
It is courteous to let a university know you will not be enrolling. If you cannot find your card do send an e-mail immediately to the SC admissions office. There are many, many spring admits waiting for that precious spot.
It is also good manners to thank the university for your offer of admission, particularly if you were offered financial aid, received an interview or a special tour.<br>
There is a thread on CC now discussing how to decline an offer of admission.</p>
<p>Eh, for a few of my other schools, there wasn’t a decline button, either, so I ended up just emailing admissions. USC isn’t the only one without the online decline link.</p>