You don't know WTF you're talking about. The RLA covers employees of airlines, which Express couriers are, and doesn't require that they be "airport employees."
Oh? OK then. They're still covered by the RLA because this imaginary "truck drivers that aren't airport employees" division works exclusively for the benefit of an RLA carrier and provides services essential to that carrier's operation.
You people need to get off that short bus that carries you to the school of "The RLA only applies to those airline employees who are directly involved with flight operations and isn't supposed to cover any truck drivers." You don't know what you're talking about.