I had posted this on another thread of someone asking the same info. Here is my experience, hopefully it's helpful.
The first two weeks of September DH and I stayed at the POP resort and we didn't have a car. We went to both Universal/IOA and Seaworld. We used the Mears shuttle which cost us $15/person round trip. You have to call the night before or no less than 2 hours in advance and have a choice of pickup times like 6:25am, 7:25, 8:25...... You can book your time for return when booking night brfore or we found it easier to go to the park, look at map and guesstimate what time we wanted to return to the hotel and call 2 hours prior from the park. Be advised that sometimes, if there isn't enough people for a shuttle van or bus, Mears will send a taxi to pick you up. The cost is still 15$/person RT and the taxi driver SHOULD give you a voucher for the return trip. I say 'should' because the taxi driver that took us from POP to Seaworld tried to rip us off. The meter read $21.50 when we got there but on his monitor screen from dispatch it said my name and a total owed $30. The driver insisted we pay the $30 but refused to give us a voucher for the return trip and claimed to not know what I was talking about. He suggested I call Mears when getting into the park which I did and reported his cab # (also be sure to write down the confirmation # they give you at time of booking). Needless to say the return trip didn't cost us anything. As for going to Universal/IOA, a bus picked us up from the hotel at 8:25am then went to a few other hotels to pick up other guests. The bus stopped at Seaworld then went on to Universal. We called from the park for pickup time and was returned in a taxi. I think the meter read something like $35. I would say that if there is only 2 or people in your party then the Mears shuttle is the way to go but if your group is larger a taxi/van would be cheaper.
We also used Mears to go to Kennedy Space Center. The bus goes on Mondays, Wednesdays and Fridays and picks you up around 8:25am and leaves KSC no later than 6pm. The bus driver is named Jaques and he is quite knowledgable and entertaining. I'm not sure of the cost of this as we prepaid through TA and had a voucher.
The shuttle to Busch Gardens in Tampa was $10/person RT, but the closest pick up spot to POP is the Premium Outlet Center which you had to take a taxi to get to. We chose not to do this and go to Seaworld instead. Petting the dolphins was worth the price of admission.