I wonder if anyone's been in this situation and what to do:

I head back to the States soon after traveling in South America for several months on a ticket I bought with miles. When I bought the ticket, I did a simple roundtrip LAX-CUZ. I'm now in Lima so I don't need the CUZ-LIM leg of the trip back. This isn't so much changing my entire ticket but just cutting off the short first leg of it. If I check in and print out my boarding pass for this first leg and just don't get on, will I get kicked off the second, international leg of the flight (LIM-LAX)? Or do I really need to pay the $150 to do it officially?

I know $150 isn't much in the grand scheme of things, but my last known paycheck arrives in April and I'm trying not to be wasteful.