You can add domestic sectors onto an already ticketed amex 2for1 booking providing it is done at least 24hr before departure. You must call to do this and there will be a fee of gbp30 pp . The tax,fees and charges levels will be re assessed and if they are higher,which they will be, then the difference would also need to be paid. The gbp30 service fee cannot be waived as this is not a transaction that is supported on ba.com.