I think your use of the phrase "featured updates" gives it away I'm afraid. There is no such thing, there is an updates section which will provide for free updates to any App Store purchase you have already made, and also (on the 'front') page) a "featured" section displaying featured apps. You've had a mixup in what is being offered, and purchased by accident.
This Featured section is not personalised to you in any way, everyone sees the same one (App Store location dependant I imagine), and it's quite possible for it to show programs that you already own.
The key is that the App store only really knows about stuff you have bought from the App store, as indicated by the fact that the purchases are tied to your Apple ID. Exceptions to this rule exist (iWork apps etc can show up as "installed" even though they were not installed through the App Store, but they cannot be updated, or redownloaded), but it's perfectly possible and indeed normal to own and even have installed a piece of software that originated from outside the App store that it simply does not know about. These could be versions of apps purchased direct from the developer, or in your case a copy of Lion that is tied to your Macbook rather than your Apple ID.
This is an important distinction to make, and one you have not fully realised after purchasing Lion effectively for a second time via the App store.
A few things to note about your purchase, now that it's been done (I'll leave the other answers to discuss how to attempt refunds etc, as they seem to have it covered)
- Your App Store purchase, unlike the version that came with your Macbook Air, is licensed to your Apple ID, and you can use it on any other Macs in your household, so if you have other machines a single purchase can be used on them all. Your pre-installed copy does not allow for this
- You are only downloading an Installer for Lion. It will not install unless you run it. Even if you run it, it will not materially affect your install - it would only be a waste of time effectively overwriting your OS files with identical ones
- Since you have downloaded it anyway, note that machines that have Lion pre-installed do not provide you with restore media, and they expect you to use internet recovery anyway if you require to access the install media in future. As you have a copy downloaded, you can store this on a DVD or Memory stick, which will speed up restore times in the future if it is ever required. You can create bootable disc or USB media from the download too, to further improve your future recovery options.
So, although you made a mistake in purchasing it, it's not the end of the world, and for $30 if you are unable to get a refund I would not be too upset so long as you take advantage of the situation by making the best of it and bearing the above points in mind.