I purchased the Expandable Cat/Dog Carrier to fly with my 12lb Dachshund and it worked wonderfully. I flew American Airlines and while the bag was a tight fit under the seat (it is a inch or two larger than what they require) it did fit and left my dog plenty of room to move around in. I found that when flying with an animal no one is standing there with a ruler to make sure your bag does not exceed the required size, rather they are more looking at can your dog move around in the carrier. If your dog can and your bag is not obviously too big they let you fly.
