Yea, you're obligated to buy it if they'll give it to you at or below the price you offer. As long as your travel plans are flexible, it should be fine. And by flexible, I mean that although you can chose the dates of your flight, you can't chose how many layovers etc. so you might end up on a trip that takes 10-12 hours on a flight from, say, florida to colorodo, which if it was a direct flight would only take 3 hours or whatever. If you want to be able to pick a direct flight, I'd reccomend checking out Southwest or one of the other cheaper airlines out there.