I'm a little late to the party since the OP asking about transportation is probably on her way now, but I just wanted to put in a plug for the
Dreams Unlimited package found here:
http://www.dreamsunlimitedtravel.com/universal/tickets.htm
I'm not sure how much advance time you need to order tickets this way, but we purchased a 2 day/2 park ticket with round trip transportation on two days when we were there in April. The shuttle picked us up from Pop and made one other stop at Bonnet Creek and we were on our way. It was a really early pick up because we wanted to be there to open the park. For both pick ups, the shuttle driver was prompt, polite and friendly. On the return trip, we were in a motor coach that was almost full. On one return trip, which was late -- after we saw the Blue Man Group, we went straight to Pop and then the bus dropped off the others. On the second return trip, which was in the mid-afternoon, we stopped at Sea World first to pick up more passengers and then the first drop off was Pop.
Anyway, buying the US/IOA tickets this way made the transportation pretty much free. I thought it was a good deal.