The $20 option is probably paying for Mears. The current cost for Mears shared shuttle service is $21 per adult o/w or $34 R/T. Guests eligible to use DME won't be using Mears. It's a per person charge.
The flat rate charged by a car service is generally less then the metered rate charged by a taxi. Many car services offer a complimentary grocery stop. R/T car service is around $100-$120 + tip.
Another option is to rent a car. You can rent it for your entire trip or rent it one day/one way with a drop off at a WDW area location.
edited to add Mears has a taxi fare estinator on their website. The estimated fare from MCO to a couple of WDW locations is $50-$55. The flat rate charged by a car service is likely to be less then the meter, assuming you don't need a grocery stop. Add a grocery stop and the car service will definitely be cheaper.
