Stop Copay Overpay Act This bill prohibits the Department of Defense (DOD) from charging a covered individual (i.e., individuals enrolled under the TRICARE program) a co-payment exceeding a certain rate for an outpatient visit for mental health or behavioral health under the TRICARE program, regardless of whether the visit is furnished by a specialty care provider. Specifically, the co-payment amount may not exceed the amount of a co-payment that would be charged under the TRICARE program for an outpatient visit for primary care services. During the one-year period beginning after this bill is enacted, DOD may not increase the co-payment amount charged to a covered individual above the amount that would have been charged in FY2021 for any service (1) furnished by a specialty care provider under the TRICARE program, and (2) that is not an outpatient visit for mental or behavioral health care.