There are airlines that charge you for each carry-on that you have. I know Spirit Airlines does this
Frontier does this for some passengers, though I think there are still ways to avoid the fee that don't involve paying more for your seat.
On a recent flight, I overheard that Spirit had bought Frontier. Tried googling it just now...turns out that Frontier was purchased by a private-equity firm headed up by a former chairman at Spirit. Given that my first (and last) experience with Spirit was total suckage in nearly every possible way (they canceled the return flight and had everyone fend for themselves until the next morning...my wife and I would've been stuck at LAX overnight if her brother-in-law hadn't gotten us out of there and back), this doesn't bode well for Frontier.