Yeah... I just went through the process and put in my AMEX. It was around $10 in fees but I didn't have any issues. It was on Jan 26th.
I'm back on their site now. I had stopped reading after the beginning of the paragraph. Thanks for mentioning you used Amex cause I'm gonna try it. Here is the verbage, which is confusing, but looks like may work:
The following Counties accept MasterCard, Visa, Discover as payment options when titling and registering a vehicle: Adams, Alamosa, Arapahoe, Archuleta, Baca, Boulder, Broomfield, Chaffee, Cheyenne, Clear Creek, Conejos, Costilla, Custer, Delta, Denver, Dolores, Douglas, Eagle, El Paso, Fremont, Garfield, Gilpin, Grand, Gunnison, Huerfano, Jackson, Jefferson, Kiowa, Kit Carson, Lake, Larimer, Las Animas, Lincoln, Logan, Mesa, Moffat, Montezuma, Montrose, Morgan, Otero, Ouray, Park, Phillips, Pitkin, Prowers, Pueblo, Rio Blanco, Rio Grande, Routt, Saguache, San Miguel, Summit, Teller, Washington, Weld and Yuma accept Master Card, Visa, Discover and American Express as a payment option when titling and registering a vehicle.