I looked at Go! Airport Shuttle, as it worked well for us 8 years ago to Universal. We are a family of 7 and I think it was $130 each way for private van service.
Looked at them a month or so ago for our 3/7-3/11trip and had same rate for POP! to MCO, but wasn't ready to book. Looked last week and it was $153. Went to book yesterday and it was $180!!! So maybe if you book Go! Airport Shuttle a few months in advance you may get a decent rate for private car/van.
In my double checking and googling for coupon codes and such, I inadvertently landed on mearstransportation.com. The interface is different than mearsconnect.com. I got private van service for $106 flat rate. Take a look there and see if it's different. Honestly, for 1 or 2 people and if I had time, I'd just do the $16.50 one-way with Mears Connect.
We got a minivan through Costco for a little over 28ish hours for our arrival, as we are traveling to the coast to visit and spend overnight with family our 1st two days. Picking up at MCO and dropping off at Doubletree on Hotel Plaza. Taking POP! bus from DS to begin our WDW portion. $277