We always use Mears. They charge $19.00 round trip/person. You can either pick up the vouchers at the Mears desk in the airport when you first arrive in Orlando or wait and call the day before you want to go to Universal. The Mears desk is right next to the DME desk. If you do get the vouchers beforehand at the airport, you can pay cash or use a credit card. The vouchers can be used any day. Just call Mears the day before you want to go to Universal and arrange a pick up time. When the van driver arrives give him your voucher, he'll tear off 1/3 and give you 2/3's. About an hour before you're ready to leave Universal give Mears a call to arrange a pick up time. Give the driver the voucher again and he'll tear off another 1/3 and give you the final 1/3 for your receipt. The $19.00 does not include gratuity of course.
If you don't want to get the vouchers at the airport, you can just call Mears the day before you want to go to Universal and arrange a pick up time. When the driver arrives you'll have to give him exact cash though. He will not be able to take a credit card. The process is the same though. About an hour before you get ready to leave Universal, give Mears another call for a pick up time. The Mears number is: 407-423-5566.
We've used them several times and have never had a problem. They may make a stop or two at another resort, but if they do, it just takes a few minutes. We've always arrived at Univeral well before opening.