I've done this route a few times, for different reasons. When I did the run for E3, the morning SD->LA run was almost empty. The afternoon LA->SD run was completely booked up and I got to be the lucky one to stand for almost an hour. I know this is opposite of what you're planning, just letting you know that the "danger" is real.

I did the same run, but this time in BC coming from the Airport and it was great. Everything was just a LITTLE nicer, which, after a 20+ hrs of traveling was VERY welcome.

In the end, does the $19 make or break it for you? For me, the $19 was well worth spending to know I'd have an easy time getting to a seat, and having some small refreshment.