The technical answer is that everyone quoting you are shipper/brokers who by definition do not own trucks, so it is important to know who your carrier is and what they will allow.  You see, carriers that do own trucks are not licensed or insured for personal items.  And brokers who do not own trucks cannot authorize personal items.  Only the carrier can authorize personal items.

Many carriers have the following disclaimer: We do NOT allow customers to place ANY personal items in vehicle during shipment, all items must be removed prior to pick up. If any items are found in vehicles they can/will be removed. We are under very strict law that we must follow with the FMCSA regarding hauling ONLY autos not household goods, we are not licensed or authorized. Not to mention items have been known to break windows and cause damage to other vehicles as well. We as well as DPS/Highway patrol also hold the right to confiscate or inspect any and all items in a vehicle. All compartments must be unlocked during transport for inspection.

That being said: Most carriers will allow up to 100 pounds in the trunk, they typically will either charge for more weight or not accept a vehicle with over 100 pounds in the trunk. Carriers rarely allow items in the interior of your vehicle because they are not licensed for personal items and their insurance does not cover your personal items. Please note that if a thief were to break the windows to get a box or any other personal item, it will not be covered by insurance. In those cases, no one is happy and your car is damaged.