I have two teapot yorkies, and I fly with them all the time. We've flown Delta with them almost every time. I believe Southwest is the least expensive. They obviously won't fit in the same bag, so my husband takes one and I take the other. I have tried so many different carriers ranging the spectrum (Juicy, Coach, PetFlys, Sherpa, etc...), and wouldn't you know it the Sherpa (cheapest!) is our favorite by far.
My girls are about 10 and 12 lbs, and they both fit comfortably in their own size medium Sherpa. The Sherpas are nice because they are a bit more flexible than some of the harder sided carriers, so you can compress them slightly to get them under the seat, whereas with some of the hard sided carriers you have to actually rotate them t their sides. I saw someone else recommended Dog Jaunt to you, which is an awesome blog, and if you do some reading there you'll see the Sturdibag also comes highly recommended. I have no first hand experience with it, but I'd trust their recommendation.
I've never had anyone check to make sure they can stand up in their carrier, but think of it like standing up while drinking water. I can't imagine there are many dogs small enough who can stand upright fully in a carrier unless it just dwarfs them.
My girls both usually just sleep on the plane, and I've never noticed them acting like their ears are bothered.
One warning though...those long distance trips in the carrier can be murder on the hair!
We were so excited the first time we tried the Sherpas, and they fit so easily under the seat (seat height does vary by type of plane) that we took a picture!