I believe there is a cargo ship under construction to service St. Helena at the new jetty pier:
http://awshipmanagement.com/2016/03/...-to-st-helena/. Air freight is not an economical way to supply St. Helena. Only high value cargo like seafood exports would be feasible. The high gross weight ERJ 190 looks like it can take a 24,000 lb. payload into St. Helena with full fuel.