So I went to the California Vehicle Code online and culled out this info ....
V C Section 259 Collector Motor Vehicles
Collector Motor Vehicles
259. "Collector motor vehicle" means a motor vehicle owned by a collector, as defined in subdivision (a) of Section 5051, and the motor vehicle is used primarily in shows, parades, charitable functions, and historical exhibitions for display, maintenance, and preservation, and is not used primarily for transportation.
Added Sec. 1, Ch. 107, Stats. 2004. Effective January 1, 2005.
V C Section 260 Commercial Vehicle
Commercial Vehicle
260. (a) A "commercial vehicle" is a motor vehicle of a type required to be registered under this code used or maintained for the transportation of persons for hire, compensation, or profit or designed, used, or maintained primarily for the transportation of property.
(b) Passenger vehicles and house cars that are not used for the transportation of persons for hire, compensation, or profit are not commercial vehicles. This subdivision shall not apply to Chapter 4 (commencing with Section 6700) of Division 3.
(c) Any vanpool vehicle is not a commercial vehicle.
(d) The definition of a commercial vehicle in this section does not apply to Chapter 7 (commencing with Section 15200) of Division 6.
Amended Sec. 11, Ch. 861, Stats. 2000. Effective September 28, 2000. Operative December 31, 2001.
Amended Sec. 1, Ch. 222, Stats. 2002. Effective January 1, 2003.
V C Section 471 Pickup Truck
Pickup Truck
471. A "pickup truck" is a motor truck with a manufacturer's gross vehicle weight rating of less than 11,500 pounds, an unladen weight of less than 8,001 pounds, and which is equipped with an open box-type bed not exceeding 9 feet in length. "Pickup truck" does not include a motor vehicle otherwise meeting the above definition, that is equipped with a bed-mounted storage compartment unit commonly called a "utility body."
Added Ch. 13, Stats. 1991. Effective February 13, 1991.
Amended Ch. 928, Stats. 1991. Effective October 14, 1991.
Amended Sec. 8, Ch. 652, Stats. 1997. Effective January 1, 1998.
V C Section 410 Motor Truck
Motor Truck
410. A "motor truck" or "motortruck" is a motor vehicle designed, used, or maintained primarily for the transportation of property.
Amended Ch. 272, Stats. 1993. Effective August 2, 1993.
V C Section 465 Passenger Vehicle
Passenger Vehicle
465. A "passenger vehicle" is any motor vehicle, other than a motortruck, truck tractor, or a bus, as defined in Section 233 , and used or maintained for the transportation of persons. The term "passenger vehicle" shall include a housecar.
Amended Sec. 1, Ch. 1008, Stats. 1999. Effective January 1, 2000.
V C Section 34710 Passenger Vehicle Defined
Passenger Vehicle Defined
34710. As used in this division, "passenger vehicle" means any motor vehicle defined in Section 465, except any of the following motor vehicles:
(a) Motorcycles.
(b) Housecars.
(c) Specially constructed vehicles.
(d) Motor vehicles equipped with four-wheel drive.
(e) Motor vehicles constructed on a truck chassis.
(f) Motor vehicles operated for hire, compensation, or profit.
(g) Makes of motor vehicles of a model year manufactured or sold in California in quantities of less than 2,000 units for each such model year.
(h) Motor vehicles designed and constructed by the manufacturer of such vehicles, for off-highway use, as determined by the Department of Motor Vehicles.
Amended Ch. 635, Stats. 1974. Effective January 1, 1975.
V C Section 9400 Weight Fees for Commercial Motor Vehicles with Declared Gross Vehicle Weight 10,000 lbs or Less
Weight Fees for Commercial Motor Vehicles with Declared Gross Vehicle Weight 10,000 lbs. or Less
9400. Except as provided in Section 9400.1, and in addition to any other registration fee, there shall be paid the fees set forth in this section for the registration of any commercial motor vehicle that operates with unladen weight. Weight fees for pickup trucks are calculated under this section. Whenever a camper is temporarily attached to a motor vehicle designed to transport property, the motor vehicle shall be subject to the fees imposed by this section. The camper shall be deemed to be a load, and fees imposed by this section upon the motor vehicle shall be based upon the unladen weight of the motor vehicle, exclusive of the camper.
[Table shows the weight fee for non-electric powered commercial motor vehicle with declared gross vehicle weight of 10,00 pounds or less equipped with no more than two axles.] *
Unladen Weight Fee
Less than 3,000 lbs. $ 8
3,000 lbs. to and including 4,000 lbs. 24
4,001 lbs. to and including 5,000 lbs. 80
5,001 lbs. to and including 6,000 lbs. 154
6,001 lbs. to and including 7,000 lbs. 204
7,001 lbs. to and including 8,000 lbs. 257
8,001 lbs. to and including 9,000 lbs. 308
9,001 lbs. to and including 10,000 lbs. 360
V C Section 9406 Alterations or Additions to Vehicles
Alterations or Additions to Vehicles
9406. Alterations or additions to registered vehicles for which fees have been paid under Section 9400 or 9400.1 placing the vehicles in weight fee classifications under Section 9400 or 9400.1 greater than the weight fees previously paid shall be reported to the department and at the same time the difference between the weight fee previously paid, reduced as provided in Section 9407, and the greater weight fee, reduced as provided in Section 9407, shall be paid to the department upon the operation of the vehicles in the greater weight fee classification under Section 9400 or 9400.1.
Amended Sec. 51, Ch. 861, Stats. 2000. Effective September 29, 2000. Operative December 31, 2001.
Its enough to make your head spin 
I just wonder if one could get out of paying a weight fee given the definitions of what a pick up is, what a commercial vehicle is and what a passenger vehicle is ? After all these are just toys for most of us.