I heard back from Mears. What they said was that I need to buy a RT ticket ($19) and only use the return trip on it. They don't sell one way tickets. When I asked if I could spread the RT out over several days, they told me it has to be the same day. So basically, you buy a RT ticket and just use one portion of it.
They also said I need to call a couple hours beforehand to schedule the shuttle. Since I'm not from the US, I have no phone, hence no option to call when in the park. When I asked whether I could schedule the return in the morning or the night before (since they do have fixed times, always 15 mins after the hour until 3am), they said no, I will need to do it on the day I need the shuttle after noon. I explained that I have no phone and finding a pay phone at Universal probably will be a hard task but the lady wouldn't budge. I have to do it on the day I want to use and a couple hours before I want to use it.
Seriously, I have never seen a company so unflexible in my whole life. I was trying to give their shuttle from the airport a go this year (I usually take Super Shuttle as I always take them, no matter which city). However, I am gonna rethink that now.