OP, call USC tomorrow. There is a liaison in the Admissions Office who deals with QB:
http://www.usc.edu/admission/undergraduate/findyourcounselor.html
Ask. I know that for our S (QB finalist) the school used a specific transportation service, and instead of taking a (cheaper) train we had to use a more expensive one, and make the reservation through a portal. They paid for it in a check at the time he arrived on campus for the program. Also be sure you are understanding the invitation, since often different administrative offices at the same school have gaps in communication. It is probably not necessary for the Match scholarship (see below) since the website specifically says that an interview is not necessary for QB Match. However it would be great for you to do it, and if your liaison suggests it then definitely make every effort!
If you are supposed to make your own arrangements, find a relative or friend with a credit card and get your ticket. Also be sure you know how you are getting from the airport to the campus. This expense would probably also be covered. Save a receipt.
This is one of those situations where it is hard for low-income students to overcome hurdles. But OP keep your eyes on the prize: http://www.questbridge.org/southerncalifornia-financial-aid