Ariz. Rev. Stat. § 40-374
Except as otherwise provided in this chapter, no public service corporation shall charge, demand, collect or receive a greater, less, or different compensation for transportation of persons or property, or for any product or commodity, or for any service rendered in connection therewith, than the rates, fares, tolls, rentals and charges applicable to such transportation or product, commodity or service specified in its schedule on file and in effect at the time, nor shall any public service corporation refund or remit, directly or indirectly, in any manner or by any device, any part of the rates, fares, tolls, rentals and charges so specified, nor extend to any person any form of contract, agreement, or any rule or regulation, or any facility or privilege, except such as are regularly and uniformly extended to all persons and except upon order of the commission.
A.R.S. § 40-374