I am guessing that is why it requires pre-booking rather than running as a "hail on the street" service. Plenty of other examples of Routemaster operated services but they require some form of advance reservation / advance purchase of a ticket with pick ups at defined points. In other words they are not normal registered passenger carrying services.
No-one could accuse them of charging rip-off fares, if they really are going to be less than £3, but I do wonder how much pre-booking will be done. Without that element working well, then I can't see a regular service operating throughout the year. I wish it well, though.