Also got advise from NBAA. They said 135 flights (unscheduled) coming from south of 30* N would have to clear according to 19 CFR Part 122 (at airports designated in FL & KILM) like private aircraft. Overflight permit would be required to land at another landing rights airport. Maybe the customs officer I talked to misunderstood my questions, because he contradicted this. Also had one FL charter operator (dispatch) tell me they could fly from Turks & Caicos direct to South Carolina to clear Customs there (2 days before requested flight). I thought they couldn't, but they argued the point.