What are the factors in determining transportation rates among different commodities?

Some of them are:

the density of the product. A product that is more dense is less expensive to ship because it takes fewer trucks to carry the same weight.

whether the product is solid, liquid or gas.

whether the product is hazardous. Hazardous materials shipments cost more than non-hazardous shipments because of insurance and the need to hire a driver licensed to haul hazardous materials.

the value of the commodity. A more-expensive commodity costs more to ship because it costs more to insure.

whether the commodity is perishable. A product that has to be refrigerated costs more to ship because they don't give reefer fuel away, and because a refrigerated trailer cannot carry as much freight as an unrefrigerated one (the refrigeration unit weighs several hundred pounds).