Regency Transportation, Inc., operates a freight business throughout the eastern United States. Regency maintains its corporate headquarters, four warehouses, and a maintenance facility and terminal location for repairing and storing vehicles in Massachusetts. All of the vehicles in Regency’s fleet were bought in other states. Massachusetts imposes a use tax on all taxpayers subject to its jurisdiction, including those that do business in interstate commerce, as Regency does. When Massachusetts imposed the tax on the purchase price of each tractor and trailer in Regency’s fleet, the trucking firm challenged the assessment as discriminatory under the commerce clause. What is the chief consideration under the commerce clause when a state law affects interstate commerce? Is Massachusetts’s use tax valid? Explain.