^^ This is advice from a Part 121 perspective, correct? The OP is asking about 135. The aircraft in question (BE-1900) does not have a “jumpseat” in the context of 121.581. The BE-1900 is not a transport category aircraft and no FSB Report exists therefore no “jumpseat” has been evaluated. The 1900, type certificated for (1) pilot, may have a right hand pilot seat installed however. One question that the OP should ask his CP is, Would said company’s insurance Policy cover the company in the event of an accident where the “extra occupant” was present?