Excuse me if I'm wrong but Mededicare Part B pays 80% of the office visit charges, not a set co-pay. If the office visit charge is $100 then Med-B pays $80...........and your mom pays the remaining $20. If the office visit charge is $200 then Med-B pays $160 and your mom pays the remaining $40.
This was about 9 years ago so I may have confused co-pay with the 80% but whatever it was I figured it up and he was wrong on the way he was billing. The person I spoke to at Social Security told me he should not be billing the way he did. Nothing ever happened as far as I know.