At least they allow us to do it. I wanted to buy an extra seat on Southwest a few years back for a flight from RNO to LAX. I was travelling at the Business Select fare, and I thought that an extra empty seat would be a nice way to complement that service. Southwest told me that they do not sell extra seats for passengers' convenience. The only two allowed circumstances are customers of size and people who have to carry musical instruments.
So why not say you have an instrument then forget to bring it? Or, say you're of size but then you had a successful diet between booking and flying?
