Keeping people from individually moving across the border from Griffonia or vice versa with pegasi was all but impossible given that, well, they could fly wherever they damn well pleased. Fortunately, policing the border was still possible because of one simple reality: shit was heavy. Therefore, transporting goods across the border, legally or otherwise, was always going to be a hassle. To ship anything of meaningful number would require flights of courier armies carrying stuff across, which was most certainly not cost efficient. If one wished to carry it by sky carriage, it would require teams of griffon or pegasi, and the concentrated weight would require the use of favourable wind currents to avoid turbulence. That made projecting most likely points of crossing and entry literally as easy to predict as the weather, which was evidently a privatised industry in this world. They might not be able to control the wind, but they could damn well tell which way it was blowing. In all, controlling the flow of goods was still very possible and manageable.