Here's what I know, or think that I know, about this area. You're a pencil salesman for XYZ pencil Co., and you're flying your plane to an office supply convention. You are going to give away pencils at your company's booth. That's okay, and a private license is all that's required. If, however, you are going to deliver a case of those pencils to a client at the convention for compensation, such as a payable invoice, you are now delivering cargo. You need not only a commercial certificate, but you must also be a 135 cargo operator. The commercial gives limited, and I mean LIMITED rights to a pilot, such as banner towing and air tours (returning to the place of takeoff), and the transport of cargo generally requires the 135 certificate.
That said, many companies operate aircraft in furthurance of their business under part 91. Most often, these companies can haul their OWN cargo under part 91, but no one else's. Their pilots are generally company employees who hold commercial certificates, since they are being paid for their piloting services to their company. The real crux of your biscuit lies in whether or not you are employed by your company AS a pilot, and thus the requirement for the commercial license, along with the second question of whether a person with a private license who flys a company owned airplane is considered to have crossed the line from "incidental" business flying to "furtherance of a business" when the contents of the plane are anything beyond other people. I would ask more questions of the principal people involved, including an experienced aviation attorney.
As an aside, people are still arguing every year as to what constitutes "holding out services to the general public" as a "common carrier". Would that be a pone conversation, a poster on a bulletin board, or an ad in a newspaper? Are your close personal friends considered to be the "general public" for purposes of that part?
The best answer may lie in the case law and the FAA counsel interpretations, and my book on this is at the flight school. I hope I have helped you in some way.
