You've earned this interview, paid your fees to PharmCas and supplemental etc. You have every right to attend. If you think there is any chance that you'll attend South over your top choice if given acceptance, then absolutely you should go.
If you know that there is no way you'd go there over your first choice then the most ethical thing to do would be to call the school and thank them for the opportunity but let them know you've accepted an offer elsewhere.
For those who are waiting for an interview invite from South, I turned mine down so there's one more spot for someone! YAY!