I have posted elsewhere on this site regarding a nightmare I had with Travelocity, which I had used with no issues in the past; but once you have a problem with a cancelled flight (even with the airline cancelling, not you, which was my case) or whatever, the customer service becomes a hellish nightmare! Just Google Travelocity complaints and you will see what I am talking about. You can also probably do the same for Orbitz to see what others have experienced. I literally must have put in almost 16 hours of time, tons of postage, letters, copies, credit card disputes,filing complaints (like with the BBB, etc.) to get a quick refund - but I was very diligent and quick - left no stone unturned. I did not procrastinate. I will never book with an on-line company again after this experience. Will go directly to the airlines site, hotel site , etc. The only on-line site I continue to use is Car Rentals.com. You do not have to pay ahead, and you can cancel anytime anyhow. I recommend that everyone read Christopher Elliot's Blog about how to handle situations like these. He is a consumer advocate and he has lists of all these sites' corporate headquarters, etc., as well as details about how to rectify your issues.
So, bottom line is - book with the airlines directly and save yourself a lot of potential aggravation. We booked directly with United for a trip recently and the itinerary changed to what I considered unsatisfactory connection times, so I called a month after that and requested different flights (that I researched myself on-line) and they changed them - no problem - and no charge. Travelocity, meanwhile, for an upcoming flight we had scheduled that the airline (Lufthansa) changed said they could not put us on another flight that same day (economy class also) only the day before or day after!; if we wanted to leave the original day we would have to pay more money! They said that flight was "a different economy class"- which they could not clarify what that meant other than that is cost more! I could almost write a book, but I truly believe that Travelocity engages in "Bait and Switch" - they had already done other things with this flight in the beginning that leads me to believe that - like posting a fare on line that you cannot submit and then when you call to say you are having a problem booking on-line they tell you the fare is no longer available, but they, of course, have another flight available for MORE MONEY!
Stay away from on-line travel agencies. Book direct!