If the bags are tagged all the way to SHD, the OP can use the midterminal immigration/customs and baggage recheck, and most likely avoid 3M baggage fees.
If he has to go retrieve his bag and go landside to drop off the bag at the 3M checkin desk, he would get hit with bag fees unless he's UA elite:
"United MileagePlus Premier members are entitled to their normal baggage fee waivers based on their United status level on Silver Airways flights only in the Washington, D.C.-Dulles (IAD) network. You must include your MileagePlus Account Number in your reservation or present a valid membership card at check-in. All customers are required to pay for bags on Silver Airways flights in Florida and the Bahamas."
That would be the most important difference, IMO.

Edit: Missed the part where the OP wrote that he's UA Gold...