Cruzer let me try and allay all your fears. We find it excellent in Miami. As Blues states, a holding area first, they come and get you and literary take you all the way through to your gate. $5-$10.00 will be fine depending on the service you receive.
Now, I don’t know if you are renting a car at SXM but if you do when you return the car they will take you to the airport, so ask the person to get you wheelchair assistance and they again will take you not only to the gate but to the steps of the plane at Julianna. If not, then just ask for wheelchair assistance.
Not sure if Miami is your final destination on return but they were waiting with wheelchair and took my wife all the way through to our waiting car in the car park. I gave her $20.00 and so worth it...through customs and out in under 15 minutes and she stayed with us whilst waiting for the luggage! Regardless, find AA very good at this. Yes depends who you get to help but should be fine. Bon Voyage.