I have mixed luck with online, sometimes it works, sometimes it doesn't. Once it worked for one, tried another one and that didn't work. Now I just call.
Good luck!
We called up and they added the Ultimate Life to our existing reservation. We see the onboard credit and the excursions option now, but not the bev package yet, will give it a few more days and check again and then call again if necessary.
Closest other airport is Long Beach, but both are within 30 minutes of San Pedro. Day of week would be a possible factor for traffic. LA stinks for that. I think you would be fine, if a week day, just disembark as early as possible.