Medical billing is dumb but I'm not sure why that would be or appear fraudulent. The normal cost of the ambulance service would be $1798, medicare pays a max of ~$500 for ambulances and your mom pays 20% of that. The reason the rest of us would pay $1798 is because we essentially subsidize the medicare rates. Many insurance companies negotiate rates that are some % of the medicare rate, so they might pay 2x or whatever, and pay $1000. Medicare often has agreements that they can't bill the rest of the charge to the patient so it's 'adjusted' essentially written off.