Cancelling

I have an onsite interview with a company in Mountain View scheduled for next week. About two or three weeks ago, I accepted the onsite interview. However, the flights they have given me are very inconvenient, i.e. red eye flight back, and 6 hour flight with stops the other way. Because of this, better offers from other companies, and how the interview will conflict with my school schedule and rest, I really want to cancel. Would it be morally incomprehensible to reject the interview at this point? That is, after they have booked a flight, and its only one and a half weeks out. Assuming it is, how can I best get out of this situation, without being a total dick? I understand, that backing out now, will probably ruin any future chances I have with the company. However, I would still prefer to back out in the most professional manner possible.