First, I apologize. We're heading cross country tomorrow, so this is at the forefront of my mind as we pack and get ready to go. And it seems that it's come up a lot this week in different places, so you got the brunt of my frustrations.
As KQ said, the RSTV is a fantastic seat. It's a wearable five point harness, but due to its nature, it's got very good numbers. It's a bit annoying, which is why it's not recommended for daily use all the time, but it's great for travel. We're actually selling ours this weekend because Piper's outgrown it by torso height. We'll be traveling with a backless booster for her. Even though the backless cost me $15, I've had it for a few years, I own a bungee cord with which to strap it to a suitcase, I'm still tempted to spend the $130 on a bigger RSTV for her. That's how awesome it is to travel with. I'd go cross country with that for Piper, and Laine's Coccoro on a Traveling Toddler. Two seats with me at all times for the unforeseen times (like when the subway station in NYC was closed, so we hopped in a cab to JFK).
I'd see if you can buy a ticket for your baby. Call the airline. Some may offer an infant in seat discount, many don't. But if you're close to the travel date they may be happy to sell you the seat for nearly anything just to get something for it. Or if there are empty seats (a rarity nowadays), you can install the seat for your baby without needing a ticket. But that's not a guarantee. And these days, it's not at all likely. But with an RSTV and a Boulevard, you can put the RSTV in your suitcase (it's not used on a plane) and the Boulevard on board. If you can get an empty seat or a ticket your baby rides in the Boulevard. If you cannot, your daughter rides in it (if you install it forward facing, ask for a seatbelt extender so you can uninstall it at the other end). Then at your destination you put your baby in the Boulevard and your daughter in the RSTV.
http://www.safetrafficsystem.com/
You can see their crash tests there. Compare them with harnessed and boostered crash tests you can find on youtube. You'll see the differences.
Now, the downsides. You must "reinstall" the seat every time. Even if you don't remove it completely from the car, you need to rethread at least two parts of it. So it's not just a plop a child in and buckle and go. You work at it a bit every time. Every time she gets out of the car you need to redo it. And it's got a bit of a learning curve. It's not like a booster where you just hand it to someone and they buckle it in. Or a carseat where pretty much most people know how to buckle a child in. Piper learned very quickly how to talk people through it, though.
Errrm, that's about it on the downsides. The upsides. It's portable, it's light, it's foldable, it's got great numbers, it doesn't expire, you can use it with a lapbelt only (if you have a top tether), it's narrow.
I got ours for travel, but I used it a lot more for a tight three across situation than I did for travel. Piper took it to school with her in my last weeks of pregnancy in case I needed to call someone random to take her home for us. She thought it looked cool, and liked to wear it.
Definitely research your options a bit, see what you can do. If you cannot buy a ticket this time, I'm sure you will next, and you won't end up in an awkward and potentially unsafe situation having to deal with the tough choice of checking or renting seats.
Wendy