If you order online, and your MBA has problems, then all you need to do is take to your local Apple Store and they will fix any flaws it may have.
If you return just because you don't like it, then that's a different story. You'd have to pay a restocking fee. And you have (I think) 17 days.
My advice: Before buying the MBA, make sure it's exactly what you want. I would recommend you trying it out at the Apple store before buying it. And if your already there, you mine as well buy it in store.