You'll be fine, the baggage fee is covered by your credit card and has nothing to do with riding DME.
24 hours before your flight to MCO you will have to check-in online for your flight with United, pick your seats, print your boarding passes and select how many checked bags you will be taking. Luggage tags for the flight are usually printed at the Kiosk at the airport. Don't confuse these with the yellow DME tags that Disney provides. You will need both on your bags, one for United to track your luggage through the airport and to your destination airport and then the yellow DME tags for when you arrive at MCO so that the DME crew can grab your bags and get them to your resort.
For your return flight, check-in online for your flight 24 hours before flight time (just as you will have done for your trip down).
You should be able to check your bags through directly from your resort to your home airport, luggage tags will be printed and put on your bags at your resort and you won't have to worry about them again until you arrive at your home airport.