Car service from SFB to MCO would run you about $75.00 to MCO. If you are thinking about going to MCO to pick up the free DME you would be wasting a lot of time and effort. There is no other transportation other than car rental.
Picking up a ride at SFB to go to MCO and catch the free DME involves car service to MCO and getting dropped off on the third floor, trekking down to the first floor to DME with luggage, getting in line and on a bus to WDW. That is a waste of at least an hour, plus the travel time from Sanford. For that hour you could be in WDW and checking ii from Sanford. The extra cost from Sanford to WDW direct would be about $15.00 to $25.00 each way depending on the company you choose to get you to WDW.
The return is just as bad. Wait for the bus to go to MCO to find your car service driver. Meet your driver on the first floor, load your luggage and head up to Sanford. You would have to start from Disney about 4 - 5 hours ahead of your departure time to get to Sanford on time.